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Track Design Handbook for Light Rail Transit, Second Edition (2012)

Chapter: Chapter 10 - Transit Signal Work

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Suggested Citation:"Chapter 10 - Transit Signal Work." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 10 - Transit Signal Work." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 10 - Transit Signal Work." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 10 - Transit Signal Work." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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10-i Chapter 10—Transit Signal Work Table of Contents 10.1 INTRODUCTION 10-1  10.1.1 General 10-1  10.1.2 LRT Operating Environment 10-3  10.1.3 Transit Signal System Design 10-3  10.2 SIGNAL EQUIPMENT 10-4  10.2.1 Switch Machines 10-4  10.2.1.1 General 10-4  10.2.1.2 Trackwork Requirements 10-4  10.2.1.3 Switch Machines 10-5  10.2.1.3.1 Electric Switch Machines 10-5  10.2.1.3.2 Electro-Hydraulic Switch Machines 10-6  10.2.1.3.3 Electro-Pneumatic Switch Machines 10-6  10.2.1.3.4 Hand-Operated Interlocked Switch Machines 10-6  10.2.1.3.5 Yard Switch Machines 10-6  10.2.1.3.6 Embedded Switch Machines 10-7  10.2.2 Impedance Bonds 10-8  10.2.2.1 General 10-8  10.2.2.2 Trackwork Impedance Bond Requirements 10-8  10.2.2.3 Types of Impedance Bonds 10-9  10.2.2.3.1 Audio Frequency 10-9  10.2.2.3.2 Power Frequency 10-9  10.2.3 Loops and Transponders 10-9  10.2.3.1 General 10-9  10.2.3.2 Trackwork Requirements 10-9  10.2.3.3 Types of Loops and Transponders 10-10  10.2.3.3.1 Speed Command 10-10  10.2.3.3.2 Daily Safety Test 10-10  10.2.3.3.3 Train Location/Train-to-Wayside Communication 10-10  10.2.3.3.4 Traffic Interface 10-10  10.2.3.3.5 Continuous Train Control Loop 10-10  10.2.3.3.6 Transponders 10-10  10.2.4 Wheel Detectors/Axle Counters 10-11  10.2.4.1 General 10-11  10.2.4.2 Trackwork Requirements 10-11  10.2.4.3 Types of Wheel Detectors/Axle Counters 10-11  10.2.5 Train Stops 10-11  10.2.5.1 General 10-11  10.2.5.2 Trackwork Requirements 10-11  10.2.5.3 Types of Train Stops 10-12  10.2.5.3.1 Inductive 10-12  10.2.5.3.2 Mechanical (Electric and Pneumatic) 10-12 

Track Design Handbook for Light Rail Transit, Second Edition 10-ii 10.2.6 Switch Circuit Controller/Electric Lock 10-12  10.2.6.1 General 10-12  10.2.6.2 Trackwork Requirements 10-12  10.2.6.3 Types of Switch Circuit Controller/Electric Lock 10-13  10.2.6.3.1 Switch Circuit Controller 10-13  10.2.6.3.2 Electric Lock 10-13  10.2.7 Signals 10-13  10.2.7.1 General 10-13  10.2.7.2 Trackwork Requirements 10-13  10.2.7.3 Types of Signals 10-13  10.2.7.4 Signal Locations 10-14  10.2.8 Bootleg Risers/Junction Boxes 10-14  10.2.8.1 General 10-14  10.2.8.2 Trackwork Requirements 10-14  10.2.8.3 Types of Bootleg Risers/Junction Boxes 10-15  10.2.8.3.1 Junction Boxes 10-15  10.2.8.3.2 Rail Junction Boxes 10-15  10.2.8.3.3 Bootleg Risers 10-15  10.2.9 Switch and Train Stop Heaters/Snow Melters 10-15  10.2.9.1 General 10-15  10.2.9.2 Trackwork Requirements 10-15  10.2.9.3 Types of Switch/Train Stop Snow Melters 10-16  10.2.10 Highway Crossing Warning Systems 10-17  10.2.10.1 General 10-17  10.2.10.2 Trackwork Requirements 10-18  10.2.10.3 Types of Highway Crossing Warning Systems 10-18  10.2.11 Signal and Power Bonding 10-18  10.2.11.1 General 10-18  10.2.11.2 Trackwork Requirements 10-19  10.2.11.3 Types of Signal and Power Bonding 10-19  10.3 EXTERNAL WIRE AND CABLE 10-21  10.3.1 General 10-21  10.3.2 Trackwork Requirements 10-21  10.3.3 Types of External Wire and Cable Installations 10-21  10.3.3.1 Cable Trough 10-21  10.3.3.2 Duct Bank 10-22  10.3.3.3 Conduit 10-23  10.3.3.4 Direct Burial 10-23  10.4 SIGNAL INTERFACE 10-23  10.4.1 Signal/Trackwork Interface 10-23  10.4.2 Signal-Station Interface 10-24  10.4.3 Signal-Turnout/Interlocking Interface 10-24  10.5 CORROSION CONTROL 10-25

Transit Signal Work 10-iii 10.6 SIGNAL TESTS 10-25  10.6.1 Switch Machine Wiring and Adjustment Tests 10-25  10.6.2 Switch Machine Appurtenance Test 10-26  10.6.3 Insulated Joint Test 10-26  10.6.4 Impedance Bonding Resistance Test 10-26  10.6.5 Power and Signal Bonding Test 10-26  10.6.6 Negative Return Bonding Test 10-26 

10-1 CHAPTER 10—TRANSIT SIGNAL WORK 10.1 INTRODUCTION The objective of this chapter is to provide trackwork designers, managers, inspectors, and contractors with a basic knowledge of the terminology, requirements, devices, and coordination issues for a rail transit signal system. 10.1.1 General Typically, there are five types of light rail transit guideway configurations to be considered, each with different signaling characteristics: 1) In-Street Mixed Traffic Right-Of-Way (Streetcar) Street-running light rail systems can be operated without railway signals on a line-of-sight basis at speeds consistent with safe operation and traffic regulations. LRV operators generally must behave as if they were operating a motor vehicle, obeying the applicable traffic regulations and, of course, being observant of the locations and movements of motor vehicles and pedestrians. In this configuration, there will generally be no signaling systems other than traffic signals. The traffic signal system may be configured to provide the LRT preferential right-of-way access over cross traffic to improve speed by minimizing intersection delays. In such cases, the presence of the LRV would be detected by signal preemption devices such as overhead wire contactors, wheel detectors, induction couplers, or other vital or non-vital devices. “Priority” is more common than absolute preemption. This holds the light rail green phase longer or shortens the light rail red phase, but doesn’t give absolute preemption. 2) Semi-Exclusive Right-Of-Way This is a configuration where the LRT is within the street but train operation occurs in a dedicated semi-exclusive trackway, physically separated from other vehicle lanes by curbing or fencing except at intersections. However, train control circuitry may be provided to influence the operation of traffic, including special indications to control train movements at intersections, similar to the streetcar scenario above. Operation may still be on a line-of-sight basis, but signal systems may be provided to protect switch operations at junctions and crossovers. Access into the operating environment by motor vehicles or pedestrians is prohibited except at defined, controlled intersections. There is normally no broken rail detection where embedded track is used since derailment is unlikely with the pavement holding the rails in general alignment. 3) Exclusive Right-Of-Way Where higher speed train operation occurs in an exclusive right-of-way, trains use signal systems to avoid collisions with other trains and with street vehicles crossing the tracks. The principles of light rail transit signaling in an exclusive right-of-way are similar to railroad main line signaling methods of providing for the safe movement of trains. The track is divided

Track Design Handbook for Light Rail Transit, Second Edition 10-2 into segments called blocks. Signals keep two trains from occupying the same block at the same time and generally keep an empty block between trains that are travelling at the posted or indicated speed. Track circuits detect trains in a block. Block systems ensure train separation with safe stopping distance. Interlocked switches and crossovers protect against conflicting routes and improper switch operation. Transit signaling also provides block supervision as required for highway street operation, warning of approaching trains at grade crossings and supervising coordination with proximate vehicle traffic schemes as required for system performance and safety. Common features in an exclusive right-of-way include the following: • Power operation of track switch facing points: power on/off switches, time sequences, induction couplers, or other non-vital devices are used to improve LRV speed by eliminating stops to throw switches, thereby allowing trains to keep moving. • Block supervision (single-track, low-speed operation): similar to preemptive devices, allows an opposing train to advance without incurring schedule delay, if possible. • Block and switch protection: basic railroad signaling technology employing wayside signals, sometimes in conjunction with mechanical or inductive train stops, to provide safe operation (newer light transit systems have employed cab signals with or without train stops for continuous speed control or communication-based train control without train stops). 4) Highway Grade Crossings Grade crossing warning: based on railroad signaling technology, gates and flashers generally eliminate any need for the LRV operator to slow down to determine if a grade crossing is clear. Grade crossing traffic control based on flashing lights only or even traffic signals only have been employed, but flashers with gates are generally recognized as the most effective type of crossing warning system. Crossing warning indicators are provided ahead of the highway grade crossing to inform the train operator as to the operational status of the highway crossing equipment. 5) Yard and Shop Operational rules and small turnouts generally restrict train operations in yards to low speeds, typically 5 to 10 miles per hour. Busy switches are normally power operated; infrequently used switches are often hand thrown. Signals or indicators are provided where required and can be either integral with or external to the switch machine. Signal system architecture for a yard can be either vital or non-vital design and include track circuits. The choice of which system is most appropriate for a specific section of track is based on operational and sometimes political considerations. A light rail system may utilize different signal technologies at different locations based on these concerns. A street-running operation at relatively low speeds requires different controls than a high-speed operation on an exclusive right-of-way.

Transit Signal Work 10-3 10.1.2 LRT Operating Environment Design differences in light rail systems are primarily related to their operating environments. Since the latter can vary over a large range, the appropriate level of signal automation varies by transit agency, their operating requirements for speed and headways, and the configuration and alignment of the track system components, including special trackwork. These can vary significantly along a given LRT line. As discussed in Chapter 1, it is not uncommon for an LRT line to be very much like a streetcar along one segment of its route, but have a semi-exclusive or completely exclusive trackway only a short distance further down the track. The optimum level of signal sophistication depends on such local circumstances and is generally determined by the transit agency responsible for providing the service. While these issues have relatively little effect on trackwork, they have a significant effect on track alignment. Specifically, where the maximum diverging speed over turnouts or civil speed restrictions are enforced by the train control system, there are a limited number of speeds that can be enforced by the cab signal codes. The actual speed assigned to each code can vary from property to property, and a decision needs to be made early on in the track design as to what the enforced speeds should be. It does little good to design a curve to accommodate a civil speed limit of 45 mph [about 70 km/h] if the available speed commands are 30 mph and 50 mph [about 50 and 80 km/h]. Since the curve isn’t good for 50 mph, it would be restricted to 30 mph by the train control system. In addition to the increased travel time, if the Ea for the curve was determined based on the unattainable 45 mph, Eu would be much less than planned and an overbalance situation might exist. 10.1.3 Transit Signal System Design The system designer is obliged to consider the signaling technology available to provide the desired system operating performance at the least total cost. Within the scope of light rail transit applications, a well-established catalogue of proven technology is available. Transit signal system design must consider not only what technology is available, but also the most rational combination of equipment for a particular application. Signal systems are customized or specified by each transit system to provide safe operation at an enhanced speed. The location of signal block boundaries is based on headway requirements and other considerations such as locations of station stops, terminals, highway crossings, storage tracks and special interlocking operating requirements. Selection and spacing of track circuits for AC and DC propulsion systems are influenced by many factors. These include the degree of detection required for broken rails or defective insulated joints, the level of stray current control required, the frequency of interfering sources of power (propulsion and cab signaling), location of cross bonding, unbalance of track circuits, and the inherent advantages of various types of track circuits.

Track Design Handbook for Light Rail Transit, Second Edition 10-4 10.2 SIGNAL EQUIPMENT 10.2.1 Switch Machines 10.2.1.1 General Track switches can be operated by hand or by power. When time and convenience are important, automated switch machines are advantageous. Switch machines may be controlled from a central control facility, central instrument house or signal hut/bungalow, or by the vehicle operator. Switch machines are used on main lines, interlockings, and yards. Switch machines are used to operate switches in turnouts and crossovers in both main line interlockings and in yard tracks. In addition to switch rails, switch machines may also be used to operate movable point frogs, derails, wide-to-gauge derails, wheel crowder derails, or wheel stops. The type of switch machine selected is dependent on the type of track installation—timber or concrete switch ties, embedded track or direct fixation track, clearances, and the requisite operating parameters. For example, switch machines used in a yard are typically designed to be “trailable,” permitting a trailing movement through a set of closed switch points without damage to the mechanism. This capability simplifies the operating circuitry. 10.2.1.2 Trackwork Requirements Switch machines in open ballasted track rest on headblock switch ties and interface with turnouts through operating and switch rods. This interface is often complicated, particularly in direct fixation (DF) or embedded track, where blockouts in the concrete must be provided for proper clearance. The following elements associated with track and structure design should be considered when selecting turnout switch machines: • Size of turnout or crossover • Number of headblock ties (typically two) • Size, height, width, spacing, and length of headblock ties • Configuration of switch rods and accessories • Thickness of number one rod • Type of basket on number one rod • Distance from centerline of switch machine to gauge line of the nearest rail • Types of tie plate for number one and two ties • Tie or mounting spacing between switch machine rods • Type of derail or wheel stop or wheel crowder • Location of mounting of switch machine, horizontally and vertically, relative to gauge line • Insulation of trackwork switch, basket, and tie plate • Switch throw distance • Location of extension plate mounting holes and interface plate • Lubrication of switch plate and track layout • Rail section and weight • Location of switch heater elements or type of switch heater (electric, hot air, or gas) • Types of switch points • Type of switch machine used in existing system—as related to spare parts and employee training for maintenance and adjustment • Inserts installed in concrete switch ties for mounting bolts of the switch machine

Transit Signal Work 10-5 10.2.1.3 Switch Machines Track switches need some sort of mechanical device to change the orientation of the switch from the straight movement to the diverging movement and back and to also hold the switch points in the desired orientation. There are two broad categories of such devices: • Switch stands, which are manually operated devices with no direct connection to the train control system. These are typically used only on seldom-used tracks that do not have any sort of signaling system. See Chapter 6 for additional discussion on switch stands. • Switch machines, which are switch-throwing devices that do have a connection with a train control system. In most cases, these are power-operated devices although they usually incorporate a means of manual operation in the case of a system failure. These devices are discussed further in the paragraphs below. Several terms are useful for understanding train movements through the switches of an ordinary turnout: • A facing point movement is one heading in a direction from the switch point toward the frog, and a train can be taking either the straight track or making a diverging movement. • A trailing point movement is one heading in a direction from the frog to the switch points with the tracks converging upon each other. − The “Normal” position of a switch is usually, but not always, with the switch set to accommodate a straight facing point movement. − The “Reverse” position of a switch is usually, but not always, with the switch set to accommodate a diverging facing point movement. − A Spring (or spring-back) switch is a switch that is ordinarily in one orientation but will permit a trailing point movement from the other track and automatically revert to the original normal position after the rail vehicle wheels have passed. Spring switches are not normally installed in main track because the point rails cannot be locked in position. With no lock, either a switch malfunction or improper train movement might lead to derailment. Where spring switches are used, strict operating rules and signal systems to verify point closure are recommended. 10.2.1.3.1 Electric Switch Machines Electric switch machines are common for light rail operations because of the ready availability of electric power throughout the system. Electric switch machines are rugged, reliable units designed for any installation where a reliable source of electric power is available. They are available in a variety of operating speeds and motor voltages. Electric switch machines may be used in main line and yard service. For installations where extra vertical clearance is needed for a third-rail shoe or vehicle clearance or on-board train control equipment, a low-profile electric switch machine can be used.

Track Design Handbook for Light Rail Transit, Second Edition 10-6 Switch machines are usually specified to meet the requirements of the AREMA Communications & Signaling Manual, Part 12.2.5, Load Curve Figure 1451-1 and thereby provide ample thrust to operate the heaviest of switches. Electric switch machines are normally provided with one throw rod, one lock rod, and one point detector rod connected to the rails. They are also available with two lock rods and two detector rods for use in cases where it might be appropriate to throw the switch rails independently. The track designer and signal designer must coordinate to ensure the specifications cover supply, installation, and adjustment of these critical elements. Gauge plate extensions can be supplied that attach the switch machine to the track switch to aid in holding the adjustments of the switch machine. Electric switch machines are usually installed adjacent to the normally closed point of the switch, so that the switch rods are in tension for the preponderance of train movements. Since about 2000, electric switch machines that are entirely contained within a steel cross tie have become available. These require no headblock ties, switch rods, gauge plates, clips, basket, or lug. The in-tie electric switch machine is bolted to stock rail, but requires a reinforcing bar on the switch point. They can be advantageous in constrained areas. They greatly simplify ballast tamping; since there are no switch rods in the ballast cribs, an automatic tamper can work straight through the switch area. 10.2.1.3.2 Electro-Hydraulic Switch Machines An electro-hydraulic switch machine can be suitable for use with virtually any type of switch point (tee rail or groove rail); wide or narrow point openings/switch throw; and standard, narrow, or broad track gauges. The switch machine can be installed at the side of the tracks or in the center of tracks, directly on ties or with rail-foot attachment. 10.2.1.3.3 Electro-Pneumatic Switch Machines Electro-pneumatic switch machines require a reliable source of compressed air. While this is economical for heavy rail transit, which features short block lengths and frequent interlockings, the economics on light rail lines usually make air power switches too costly. 10.2.1.3.4 Hand-Operated Interlocked Switch Machines Hand-operated interlocked switch machines are typically used where facing point lock protection is required to help safeguard the movement of high-speed main line traffic over a switch. These switch machines contain a locking bar that, with the switch in the normal position, enters a notch in the lock rod. This arrangement locks the switch points in their normal position to provide facing point lock protection. At layouts where additional train protection is required, a switch control controller box is installed with a point detector rod attached to the switch point. 10.2.1.3.5 Yard Switch Machines Yard electric or electro-hydraulic switch machines are simple and compact machines designed for installation in tight spaces. A common yard movement will see a light rail vehicle making a converging movement at a turnout with the switch set contrary to the desired path. Doing this with a standard main line design switch machine would wreck the mechanism. Because of this, some yard electric switch machines can handle trailing moves at maximum speeds up to 20 mph [30 km/h] (which is faster than the speed limit in most transit yards) without damage to the locking mechanism. If point detection is required, an additional circuit controller can be installed. Typical yard switch machines can be adjusted for throw, from 4.5 inches [114 mm] up to 5.5 inches [140

Transit Signal Work 10-7 mm] for tee rail and 2.4 inches [60 mm] to 4 inches [100 mm] for grooved rail. The low-profile yard electric switch is available with external switch indicator lights. Electro-pneumatic switch machines are also available for yard applications, and a compressed air plant at the yard or maintenance facility may make them economical. 10.2.1.3.6 Embedded Switch Machines Embedded (surface) switch machines are designed to throw tongue and mate, double-tongue, or flexive tongue switches. The embedded switch machine can be installed between the rails (preferred) or on the outside of the switch tongue on a paved street. Embedded switch machines can be powered from the overhead catenary (at 600 to 750 volts DC) or from a separate AC or DC source. Embedded switch machines may be all-electric operation or employ electrical systems only to control hydraulic mechanisms and power a hydraulic pump. Most embedded switch machines permit the switch to be trailed without damage to the machine; however, routine use of this feature typically causes wear on mating parts and increasing difficulty in keeping the switch machine in proper adjustment. Impressing upon operations personnel that the trailable feature is not intended for routine use can be an appreciable challenge for the transit agency’s track and signal maintainers. There are no North American standard specifications applicable to embedded switch machines. Because they are virtually never used in railroad applications, they are not addressed by the AREMA Communications & Signaling Manual. While there are some applicable requirements in European standards such as BOStrab, EN, IEC, and VDV requirements, they generally apply to the interfaces and not to the machines themselves. These machines are therefore generally designed in accordance with manufacturer’s specifications, and there are appreciable differences between makes and models. Because embedded switch machines are usually used with tongue switches, their range of adjustment is appreciably different from switch machines designed for open track. Typically, machines designed for application to flexive tongue switches of European design can be adjusted only within a range of 60 to 100 mm [2.4 to 4 inches]. Caution is advised when applying these machines to AREMA-style split switches. If the throw (measured at the switch rods) is set too low, there may be insufficient clearance between the stock rail and the switch point at the rear of the side planing on the switch rail. See Chapter 6 for additional discussion of this issue. Like all power switch machines, the switch case for an embedded switch machine must be grounded for the safety of maintenance personnel. In the case of switch machines that are energized by a tap to the traction power, this can create a stray current leak unless detailed correctly. The responsibility for that design should be with the signal system designers, but the trackwork designer needs to verify that it has been done so the trackwork isn’t blamed for a stray current leak. Drainage of embedded switches and switch machines is critical. The embedded switch machine track box should be drained to a nearby storm pipe, because an undrained box collects a mixture of sand, water, salt, etc., that increases wear on moving parts and prevents their proper

Track Design Handbook for Light Rail Transit, Second Edition 10-8 lubrication. An access/cleanout box is often installed on the field side of the switch casting to provide access to connecting rod adjusting nuts if they extend beyond the switch. 10.2.2 Impedance Bonds 10.2.2.1 General Impedance bonds are necessary when insulated track joints are used to electrically isolate track circuits from each other and broken rail detection is desired for both rails. The impedance bonds permit propulsion current to flow around the insulated joints while inhibiting the flow of signal current between adjacent track circuits. They also provide crossbonding between the two rails of the track so as to balance propulsion current for track circuits and touch potential and provide negative return to the traction power substation. The stagger between insulated joints should be 2 feet [0.6 meter] or less for transit signaling to reduce the amount of cable needed as well as the unbalance in the current in the rails associated with impedance bonds. The usual location for impedance bonds is in the gauge of the track; some railways place them on independent structures outside of the vehicle dynamic envelope. Audio frequency track circuits are separated from each other by using a different frequency in each circuit; thus, they do not normally require insulated joints to isolate the track circuits, and impedance bonds are generally unnecessary. The exception occurs when insulated joints are used with audio frequency track circuits to provide a precise definition of block limits, such as at signal locations. Where broken rail detection is not required, a single rail track circuit can be used and all of the return traction current will flow through the continuous return rail. With single rail track circuits, an insulated joint is used in the non-return rail, which is then called the signal rail. Impedance bond connections to the rails can be by various methods. See Article 10.2.11 for additional information on all types of cable connections to the rails. 10.2.2.2 Trackwork Impedance Bond Requirements The following elements associated with track and structure design should be considered when designing impedance bonds: • Tie spacing for impedance bond equipment • Location of tie or direct fixation mounting holes for signal equipment • Location of impedance bond, either between or outside the rails • Location of guard and restraining rails • Location and spacing of insulated joints • Space for cables and conduit to pass beneath the rail • Conduit and cable location for signal equipment • Block-out requirements for embedded or direct fixation • Mounting to wooden, concrete ties and direction fixation

Transit Signal Work 10-9 10.2.2.3 Types of Impedance Bonds 10.2.2.3.1 Audio Frequency Audio frequency impedance bonds are designed to terminate each end of audio frequency track circuits in transit installations. They provide the following: • Low resistance for equalizing propulsion current in the rails • Means of cross bonding between tracks to reduce electrical resistance along the negative return path • Connection for traction power negative return • Means of coupling the track circuit transmitter and receiver to the rails • Means of coupling cab signal energy to the rails • Means of inhibiting the transmission of other frequencies along the rail 10.2.2.3.2 Power Frequency Power frequency impedance bonds are designed for use in AC or DC propulsion systems that use insulated joints to isolate track circuit signaling current from signaling currents of adjacent circuits, but permit propulsion current to flow around the joints to or from adjacent track circuits. AC impedance bonds are usually rated for 300 amps per rail and DC impedance bonds are usually rated for between 1,000 and 2,500 amps per rail. Typically, power frequency impedance bonds are installed in pairs at insulated joint locations and mounted between the rails across two adjacent ties, as an impedance bond is required on each side of the insulated joints. Power frequency impedance bonds provide the following: • Low resistance for equalizing propulsion current in the rails • Means of cross bonding between tracks to reduce electrical resistance along the negative return path • Connection for negative return • Means of coupling the track circuit transmitter and receiver to the rails • Means of coupling cab signal energy to the rails • Means of inhibiting the transmission of other frequencies along the rail 10.2.3 Loops and Transponders 10.2.3.1 General Loops and transponders are used to transmit information to the train independent of track circuits. They may be found in all types of trackwork and can be used for intermittent transmission or continuous control systems. In determining the type or location of loops or transponders to be used for a light rail transit system, consideration should be given to the operation plan, type of track circuits, propulsion system, and train control system that is installed. 10.2.3.2 Trackwork Requirements The following elements associated with track and structure design should be considered when designing loops or transponders: • Location of loop or transponder inside or outside the rail • Mounting the loop or transponder to the ties and direct fixation trackway • Tie spacing and mounting method for loop or transponder

Track Design Handbook for Light Rail Transit, Second Edition 10-10 • Cable and conduit location for signal equipment • Block-out area for loop or transponder and junction box 10.2.3.3 Types of Loops and Transponders 10.2.3.3.1 Speed Command Speed command loops are used to provide a means for coupling cab signal energy to the rails. Typically, speed command inductive loops are installed with or without rubber hoses within the turnout diverging track. They may be attached to the tie or concrete or clipped to the rail. Normally, a rubber hose with wire inside is installed near the inside of the rail at interlockings and turnout switches. These loops provide isolation from the track circuits. 10.2.3.3.2 Daily Safety Test Daily safety test loops are used to provide a means for verifying the function of carborne train control equipment. Typically, daily safety test inductive loops are installed with or without rubber hoses in the yard tracks or mainline storage tracks in the field. They may be attached to the tie or concrete or clipped to the rail. Normally, a rubber hose with wire inside is installed near the inside of the rail. Newer LRT systems do not require daily safety test loops due to on-board testing on the LRT vehicle. 10.2.3.3.3 Train Location/Train-to-Wayside Communication Train location loops or train-to-wayside (TWC) communication loops are designed to provide very precise definition of a train’s location, automatic vehicle identification, and either one-way or two- way train/wayside communication. A wire loop installed between the rails and on ties links the train to the rails. The horizontal loop of the wire is directly mounted or placed in a heavy polyvinyl chloride (PVC), epoxy, or fiberglass (FRE) conduit or channel that may also be encased in pavement. These loops can vary from 8 feet to 20 feet [2.5 to 6 meters] depending on the operation. 10.2.3.3.4 Traffic Interface Loops or transponders can be used to pre-empt traffic signals or provide phasing command and release of traffic control devices. 10.2.3.3.5 Continuous Train Control Loop Typically loops between stations are transposed at regular intervals. This provides a signal to the on-board equipment that can be used to recalibrate an on-board odometer. In station areas, short loops may be provided for accurate automatic station stopping. 10.2.3.3.6 Transponders Transponders are designed to transfer data, such as the intended train routing or destination, between the vehicle and train control equipment that is located along the trackway. Transponders or antennae may be mounted overhead or on the wayside, embedded between the rails, or mounted to concrete surfaces or ties.

Transit Signal Work 10-11 10.2.4 Wheel Detectors/Axle Counters 10.2.4.1 General Wheel detectors and axle counters are used to detect trains without relying on a track circuit. Since they do not require insulated joints, they cause less interference with traction return current than detection devices that depend on electrical signals in the rails. When used without track circuits or cab signaling within the rails, they eliminate the need for insulating switch rods. However, they are unable to detect broken rails. In selecting the type and model of wheel detector/axle counter, consideration should be given to the operation and mounting method used. 10.2.4.2 Trackwork Requirements The following elements associated with track and structure design should be considered when designing wheel detectors/axle counters: • Type and size of rail • Mounting hole size • Conduit and cable location • Rail grinding • Maintenance • Block-out requirements or box requirement • Drainage for embedded housing 10.2.4.3 Types of Wheel Detectors/Axle Counters The wheel detector/axle counter unit consists of a detector head or mechanical detector arm, mounting hardware, a logic board, and interconnecting cabling. Wheel detectors and axle counters are mounted with clamps that attach to the base of the rail or are bolted directly to the web. The wheel detectors/axle counters are activated when a vehicle passes. The magnetic wheel detector/axle counter is independent of the wheel load and subjected to almost no wear since there is no mechanical interaction between the detector and vehicle wheels. 10.2.5 Train Stops 10.2.5.1 General Train stops (also known as “trip stops”) activate a train’s braking mechanism if the train passes either a restrictive cab signal aspect or a fixed signal. They can be inductive units or electrically driven mechanical units. The latter are found only on older installations and are ill-suited for areas where snow and ice can interfere with their operation. In designing train stops, consideration should be given to the location of vehicle equipment, type of trackbed, operation (directional or bi-directional), relationship to wayside signal layouts, and location of the train stop elements. Train stops are used in exclusive rights-of-way and are not applicable to mixed traffic street-running applications. 10.2.5.2 Trackwork Requirements The following elements associated with track and structure design should be considered when designing train stops: • Type of track—ballasted, direct fixation, or dual block • Tie spacing • Type of tie—timber or concrete

Track Design Handbook for Light Rail Transit, Second Edition 10-12 • Location of train stop • Conduit and cable location • Relationship to signals, insulated joints, and impedance bonds • Location of train stop and wheel detectors relative to top of rail 10.2.5.3 Types of Train Stops 10.2.5.3.1 Inductive Inductive train stops are designed with a magnetic system that interacts with carborne vehicle control equipment. Both the vehicle magnet and the track magnet need to be strategically mounted on the vehicle and track, respectively. 10.2.5.3.2 Mechanical (Electric and Pneumatic) As noted previously, mechanical train stops are generally considered obsolete technology, but are found on older systems. The key component of the mechanical train stop is the driving arm, which is pulled to the clear position a ½ inch [12 mm] below the top of the running rail by either an air or electric motor and returned to its tripping position by a spring. Mechanical train stops are usually mounted on plates midway between two rails with the operative arm outside either the right-hand or the left-hand rail. 10.2.6 Switch Circuit Controller/Electric Lock 10.2.6.1 General A switch circuit controller is a mechanism that provides an open or closed circuit indication for a two-position track appliance, such as a switch point. A mechanical linkage to the crank arm of the controller actuates its normal/reverse contacts. The switch circuit controller provides independent contacts that allow separate adjustments at each end of the stroke. Commonly used to detect switch positions, the switch circuit controller can be used to detect positions of derails, bridge locks, slide detectors, and tunnel doors. The switch circuit controller can shunt track circuits as well as control relay circuits. Electric switch locks prevent unauthorized operation of switch stands, hand-throw switch machines, derails, and other devices. In determining the rods and type of switch circuit controller/electric locks, consideration should be given to operation, type of switch or derail, mounting, and clearances. The switch circuit also protects vehicles by ensuring that the switch points are closed. 10.2.6.2 Trackwork Requirements The following elements associated with track and structure design should be considered when designing wheel switch circuit controller/electric locks: • Type of track bed—ballasted, direct fixation, or dual block • Type of tie—timber or concrete • Length of tie • Left- or right-hand layout • Type of hand-operated switch machine or derail • Number and location of connection lugs on derail • Location of conduit and cable • Dapping of tie

Transit Signal Work 10-13 10.2.6.3 Types of Switch Circuit Controller/Electric Lock 10.2.6.3.1 Switch Circuit Controller A switch circuit controller is a ruggedly constructed unit commonly used with switches to detect the position of switch point rails. The switch circuit controller has a low clearance profile and is mounted on a single tie. 10.2.6.3.2 Electric Lock An electric switch lock operates by a means of a plunger that is lowered into a hole in the lock rod connected to switch points, derails, or other devices. Another electric switch lock secures the hand- throw lever on a switch stand or switch machine in the normal position. This type of electric switch lock does not require dapping the ties and may be applied to either a right- or left-hand layout. 10.2.7 Signals 10.2.7.1 General Wayside track signals are usually light fixtures either mounted on poles or positioned at ground level (dwarf signals) next to switches. Some applications in embedded track have used the equivalent of airport runway lights mounted in the pavement between the rails. Several variations of color-light signals with various indications are currently in use on light rail systems. In determining the type and configuration of wayside signals to be used, consideration should be given to operation, clearances, signal layout, track layout, right-of-way, and insulated joint locations. 10.2.7.2 Trackwork Requirements The following elements associated with track and structure design should be considered when designing signal mast installations: • Insulated joint locations • Right-of-way clearances • Conduit and cable location • Vehicle clearances • Stopping distances • Sight lines for vehicle operators • Block out for indicators, embedded track • Drainage for signal indicator box 10.2.7.3 Types of Signals Long-range color-light signals consist of one or more light units with an 8.4-inch [213-mm] outer lens for high signals and a 6.4-inch [162-mm] lens for dwarf (low) signals. These high and dwarf signals can be equipped with lenses for improved visibility along curved tracks. Dwarf signals are designed for direct mounting on a ground-level pad such as a concrete foundation. The main line high signals have backgrounds, hoods, pipe posts, ladders, pole mounting brackets, and foundations. Compact color-light signals designed specifically for use in close-clearance transit tunnels are also made. A 5-inch [127-mm] lens signal is typically used in subway installations where space is limited, while a 6.4-inch [162-millimeter] lens signal is also available for outdoor service. Transit signals are supplied with brackets for mounting on subway walls, ceilings, or poles. These signals are available as incandescent or LED types.

Track Design Handbook for Light Rail Transit, Second Edition 10-14 Embedded signals or indicators are designed to be installed in embedded track. The housing of the embedded signal can require drainage. Yard signals or indicators are fabricated in dwarf assembly case aluminum or steel housing. These signals or indicators are available as incandescent or LED types. 10.2.7.4 Signal Locations Trackway civil design needs to provide sufficient space for signals and other types of train control system infrastructure. This includes not only space for the signal itself, but also sufficient working space so that signal maintainers can safely inspect and work on the equipment. Vehicular access to these locations is also highly desirable as the maintainer’s tools and equipment can be heavy and bulky. Signals are normally installed to the train operator’s right side when operating in the normal direction for the track. They are ordinarily located at a minimum clearance offset distance from the dynamic envelope. For convenience of field measurements, the offset dimension is frequently specified from the gauge line of the near rail instead of the centerline of track. Where insulated joints are used in territory with double rail track circuits, the signal is typically located between the two insulated joints. The signal can be moved ahead of the insulated joints to a distance no greater than the overhang of the vehicle. Signals and switch position indicators for embedded tracks are sometimes installed in low-profile “runway lights” between the rails of the embedded track. The housing of these embedded signals can require drainage. Yard signals or indicators are installed next to the switch machine side or in front of the switch machine or at the insulated joint location to a junction box or pedestal. 10.2.8 Bootleg Risers/Junction Boxes 10.2.8.1 General Bootleg risers/junction boxes provide a central termination point for signal cables. Bootleg risers/junction boxes come in a variety of sizes, with or without pedestals, and are constructed of cast iron or steel. Based on the application of the bootleg risers/junction boxes, the location can be in the center of tracks, outside or inside the gauge side of the running rail, outside the end of tie, outside the toe of ballast, or next to the switch machine or other signal appliance. In selecting the type and size of bootleg risers/junction boxes, consideration should be given to the type of trackbed, cable, signal equipment, and mounting method used. 10.2.8.2 Trackwork Requirements When designing bootleg risers/junction boxes, the following elements associated with track and structure design should be considered: • Conduit and cable location • Type of trackbed—ballast, direct fixation, embedded, or dual block • Tie spacing • Maintenance • Stray current control

Transit Signal Work 10-15 • Drainage track junction boxes for embedded track • Block-out requirements • Rail size and type 10.2.8.3 Types of Bootleg Risers/Junction Boxes 10.2.8.3.1 Junction Boxes Pedestal-mounted junction boxes are typically used in ballasted track at switch machines, switch circuit controllers, track circuit locations, etc., as a central termination point for underground cables. A variety of adapter plates allow the junction box to be used with air hose adapters and connectors during testing and when staging the installation of cabling that cannot be completed due to incomplete trackwork construction. In direct fixation track or dual-block tie track, the junction box enclosure is mounted to the invert of the track structure or on the wall of the structure. 10.2.8.3.2 Rail Junction Boxes The rail junction box is a compact enclosure for providing access to cable connections to the rail in embedded track. It could be made of either fabricated steel or a casting. The rail junction box is bolted to the web of the rail, most frequently on the field side of the rail. Stainless steel or brass connector bolts are inserted through the track rail junction box wall from the inside and are attached to the web of rail. To minimize stray current potentials, the connector bolts are insulated in the rail junction box wall. These units will normally have drainage outlets since the lids will not be watertight. 10.2.8.3.3 Bootleg Risers Bootleg risers are designed as a termination point between the underground cable and the track wire to the rail or signal device. They are available with a bottom outlet, as well as side and bottom cable outlets. A typical bootleg riser installation in ballasted track would locate the riser box either in the center of the track with the top slightly below the top of ties or just beyond the end of ties. 10.2.9 Switch and Train Stop Heaters/Snow Melters 10.2.9.1 General Switch and train stop heating systems are designed to keep rail switches, switch rods and tongues, and mechanical train stop arms free of ice and snow in a predictable and reliable fashion. In designing the heating system, consideration should be given to the type of power available, type of trackwork, type of trackbed, operation, type of train stop, type of switch machine, and mounting method used. 10.2.9.2 Trackwork Requirements When designing switch heaters and snow melters, the following elements associated with track and structure design should be considered: • Size of turnout or crossover • Length and other dimensional details of the switch rails • Ease of maintenance or replacement of heater parts • Type of rail brace with notch, if required

Track Design Handbook for Light Rail Transit, Second Edition 10-16 • Conduit and cable location • Junction box(es) location(s) • Length of switch point • Number of switch rods • Type of trackbed—ballasted, embedded, direct fixation, or dual block • Stray current control requirements • Access to switch tubular heater • Block out for embedded track and support 10.2.9.3 Types of Switch/Train Stop Snow Melters There are several snow melter systems commonly used in the transit industry. The most popular system for open track features tubular resistor electric snow melters that can be installed on either the field side or gauge side and either at the underside of the rail head or at the base of the rail. For gauge side installation, holes for heater support clips are drilled in the neutral axis of the rail using a clearance drill with 3/8-inch [10-mm] bolts. For field side installation, snap-on clamps are used (no drilling is necessary). Tubular electric snow melters mounted on the field side of the rail require the special trackwork rail brace to be notched for passage of the snow melter. The rail web heater can also be used to prevent switches from freezing. The rail web heater is a low-density panel that spans the rail web. It consumes 20 to 40% less power than a tubular heater installation. Rail web heaters are interconnected to provide more heat to the point and snapped into place using rugged clips and a special clip tool. No braces need to be loosened or grooved to allow installation, which provides for easy removal in the spring prior to track maintenance or repair. Power is supplied to electric snow melters from the overhead catenary through a snow melter control cabinet or case or from a local AC power source. Switch rod heaters are used to melt snow and ice away from switch rods. These switch rod heaters are installed in the bottom of the crib where the switch rods are located. They consist of a steel channel or panel with tubular electric heaters or a series of heating elements attached. The tubular electric heater can be mounted on a swing bracket that clamps to the base of the rail on the field side and is adjustable for all sizes of rails. For heating tongue switches used with grooved rail, the tubular electric heater can be installed either beneath the switch tongue bed or outside of the switch housing. Wherever it is positioned, the heating element needs to be well-protected from the grit that is endemic in any street environment, yet still be easily accessible for inspection and replacement. While positioning the heater tube beneath the tongue bed would seem to be the most efficient way to get the heat to the location where it is needed, access for heater tube renewals could be very difficult. To provide access for maintenance, the tubular electric heater can be installed in a steel tube attached to the outside of the switch housing. The installation detail needs to avoid the possibility of heat damaging any electrical and acoustical isolation systems that are part of the tongue switch assembly and naturally should not create a stray current leakage path. Train stop mechanisms in open track can be furnished with hairpin-shaped heaters or heating panels.

Transit Signal Work 10-17 Other types of snow melting systems include oil, natural gas, or an electric high-pressure heating unit that forces hot air throughout the switch area via ducts and nozzles. An alternate snow blower arrangement uses ambient non-heated air to blow snow clear of the switch point areas. 10.2.10 Highway Crossing Warning Systems 10.2.10.1 General Highway crossing warning systems provide indications to motorists that a light rail vehicle is approaching the crossing. Such systems are commonly but erroneously called crossing “protection” systems. That term is incorrect as there is no way to protect the train from a motor vehicle whose operator elects to ignore the signals and no way to protect the motorist from the consequences of his/her failure to comply with the signals provided. What the signals can do is warn the operator of a motor vehicle on an intersecting path that a train is approaching. Currently, there is no effective way to advise an LRV operator that motor vehicles are approaching a crossing, making it highly desirable to keep the “sight triangles” at all four quadrants clear of obstructions such as buildings and vegetation. The most common configuration for highway crossing warning systems is conventional flashing light signals, either with or without gates, such as those commonly used on freight railroad grade crossings. In determining the type and configuration of the highway crossing warning system, consideration should be given to LRV operations, type of track circuit, roadway layout and posted speeds, traffic signal(s) location, right-of-way, and clearances. The challenge of fail-safe crossing warning systems is to separate the LRV and highway traffic without closing the crossing to motor vehicle traffic for extended periods of time. The Federal Highway Administration’s Manual of Uniform Traffic Control Devices (MUTCD) now includes recommendations for at-grade crossings of LRT tracks. These requirements are included in Part 8 of the MUTCD (2009 edition), which can be downloaded from the FHWA’s website. In some cases, crossing warning systems will include specific signs and warning signals for pedestrians. The track designer needs to coordinate the layout of the crossing surface and the approach pavements so that pedestrians are directed along paths where they can clearly see warning devices. In addition to the requirements currently in the MUTCD, there have been numerous experimental installations of barriers and crossing warning devices to promote the safety of both pedestrians and motorists and, by extension, the operators and passengers of the rail vehicles. One example is active signage that flashes to indicate a “Second Train Coming” from the opposite direction of the one that initially activated the warning system. This issue is dynamic and LRT design teams are encouraged to consult recent trade publications and published papers for state-of-the-art information. Crossing warning installations should be interconnected with any traffic signals located within 200 feet [60 meters] of the highway grade crossing. Additional advance warning of approaching LRVs, i.e., more time than required at an ordinary crossing, may be required for proper operation of the traffic signals so as to “flush” certain legs of the intersection prior to train arrival. An on-site diagnostic team meeting is usually required at an early date to discuss all of the implications of the warning system being proposed.

Track Design Handbook for Light Rail Transit, Second Edition 10-18 Crossing warning systems work best, with minimal delays to traffic, when the approach time of trains to the crossing can be accurately predicted. If train speed might vary, or if there is a possibility that the train might even stop on either the approach or departure side of the crossing, vehicular traffic could be significantly delayed for reasons the motorist will not understand. This can lead to motorist disrespect for the crossing warning devices and unsafe behavior by drivers. Such problems can occur whenever either wayside signals or stations are located too close to the crossing. Ideally, such items should be located either well in advance of the crossing warning system start circuits or sufficiently beyond the departure end of the crossing to ensure that the longest train will have cleared the street and the signal circuits prior to stopping. 10.2.10.2 Trackwork Requirements When designing highway crossing warning systems, the following elements associated with civil and trackwork design should be considered: • Location of insulated joints (if required) • Location of crossing slabs • Minimum ballast electrical resistance • Tie spacing • Right-of-way clearance to highway crossing equipment • Conduit and cable location • Insulation of running rails from each other if a track circuit is used for the warning system • Drainage of trackbed, roadway, and sidewalks so that the ballast section is kept clean and dry • Sight triangle clearances 10.2.10.3 Types of Highway Crossing Warning Systems A typical highway crossing may consist of flashing light units, gate mechanisms with arms up to 38 feet [11.6 meters] long, poles, foundations, cantilever assemblies, cables, case or signal houses, junction boxes, and track circuits with island circuits. Some LRT highway crossing systems have a highway crossing system warning indicator signal (typically a “lunar light”) in advance of the crossing to verify the operational status of the highway crossing warning equipment to the LRV operator. Where the LRT is located in a street with exclusive lanes and train speeds do not exceed 25 mph, it is usually more appropriate to control the crossings with vehicular traffic by using conventional traffic signal technology as opposed to railroad-type flashing lights and gates. Special signal heads, frequently using different colors or symbols than ordinary traffic lights, provide the LRV operator with guidance as to when he may safely cross the intersection. Crossings with train speeds between 25 and 35 mph should be carefully evaluated to determine whether traffic signals will provide the sufficient degree of safety. Crossings with train speeds higher than 35 mph should use railroad-type warning systems, although, as of 2010, this topic is under study by TRB and others. Reference to the current edition of MUTCD is essential. 10.2.11 Signal and Power Bonding 10.2.11.1 General Signal and power bonding is used to establish electrical continuity and conductive capacity for traction power return and signal track circuits. It prevents the accumulation of static charges that

Transit Signal Work 10-19 could produce electromagnetic interference or constitute a shock hazard to track maintenance personnel. It also provides a homogeneous and stable ground plane, as well as a fault current return path. Power bonding is typically installed at all non-insulated rail joints, frogs, restraining rails, guard rails, and special trackwork locations. Power bonding of the restraining rails requires special attention to avoid creating run-around paths that can falsely energize the track circuit or compromise broken rail detection. There are basically two types of rail connections used in the transit industry: mechanical (electric compress, bolted, or drill pin) and exothermic welding. In determining the type and the amount of signal and power bonding, consideration should be given to type of track circuits, capacity of the traction power equipment, type of rail, vehicle wheels, and the degree of desired broken rail detection. 10.2.11.2 Trackwork Requirements The following interface elements associated with track and structure design should be considered when designing signal and power bonding: • Type and size of rail • Spaces for bonding to be installed • Space for signal and power bond passing beneath the rail • Type of trackbed—ballasted, direct fixation, embedded, or dual block • Location of rail joints, insulated or non-insulated • Location of guard and restraining rail • Signal cable connection to rail in special trackwork • Location of crossbonding and traction power negative return • Block outs for power cables for connection or junction box • Location of signal track fouling wires 10.2.11.3 Types of Signal and Power Bonding Impedance bond leads are factory made to system specifications and impedance bond type for ease of installation, eliminating a typically cumbersome field application. One method of connecting cables to rails is via plug bonds. This method involves drilling a hole in the rail and hammering the plug into the hole. Exothermic welding, on the other hand, generates molten copper to create a solid bond between the cable and rail or between cables. Bonds used to be welded to the rail by either electric arc welding or gas welding, but these methods are seldom used anymore because of the amount of equipment that needs to be mobilized and the need to provide a skilled welder. Arc or gas welding to the base of the rail used to be common, but it is not recommended due to the high chance of thermal damage to the rail. Rail bonding connections in embedded track require special consideration in part because they will be completely concealed by the pavement. It is therefore recommended that the track details provide for both the protection of the cable connection to the rail and permit inspection and maintenance to occur. A common detail is to provide fabricated steel boxes where traction power cables (for example, those going back to the traction power substation) connect to the

Track Design Handbook for Light Rail Transit, Second Edition 10-20 rails. The conduits carrying the cables will stub up into the box, which then has a lid held in place by corrosion- and tamper-resistant fastenings. Traction power bonding cables are sometimes run completely around any bolted special trackwork unit so that negative return is not dependent on a series of joint bonds within the unit. This can enhance the safety of workers that might be replacing a component such as a frog. Coordination with signal design is essential. Volume 3 of TCRP Report 71 evaluated broken rail problems that some rail transit systems were having in the vicinity of large cable bonds that had been attached to rails by the exothermic process. The general conclusion of that study was that large bond cables should be attached to the rail (particularly high-strength alloy rails) by the exothermic process only under rigorous procedures with close attention to quality control. In the absence of such controls, there is a high probability that the rail steel in the attachment zone will be transformed into an untempered martensite and thereby be subject to brittle fracture. Hence, unless exothermic bonding quality processes can be ensured, plug bond systems or bolted or compression electrical connections are the preferred method for attaching large cables to the rails. Advantages of exothermic welding versus plug bonds or bolted or compressed connections include the following: • The installation resistance of a length of exothermic weld bond is less than that of other types of bonds of the same length and cable stranding. Resistance will not change throughout the life of the bond. • Provided the rail was ground clean prior to making the weld, there should be no corrosion between an exothermic weld bond and the rail. Intermittent signal failures due to the varying resistance of a corroded rail joint will be eliminated. • Because bonding does not rely on a friction fit, bond losses caused by dragging equipment, reballasting, and snowplows are reduced. • Train traffic will not loosen a properly installed exothermic weld bond. • Rail head signal bonds that are applied within 5 inches [125 mm] of the end of rail (per AREMA Communications & Signaling Manual, 8.6.40 C.1) maximize detection of a broken rail compared to plug bonds that are applied outside of the splice bars. • Rail web bonds from 0.2 to 0.4 square inches [140 to 250 square mm] provide a convenient means of connecting all cable outside the confines of the splice bar, including special trackwork. • The exothermic weld process provides an efficient field method for any electrical connection from signal and power to ground. • Properly applied, the exothermic weld normally outlives the conductor itself. • No hole drilling is needed, and there is no chance of an oversized hole providing an inadequate connection. Advantages of plug bonds, bolted bonds, or compression bonds versus exothermic welding include the following: • The rail connector clamp can connect cables from 0.4 to 1.6 square inches [250 to 1000 square mm] to the running rails.

Transit Signal Work 10-21 • Mechanical connectors provide a rail connection without the risk of overheating the rail steel during installation. • Rail connection can be easily relocated or temporarily removed without grinding the rail or chopping the connection. • Proprietary plug-type rail bonding systems are available that have a greatly reduced probability of loosening under vibration compared with much older designs of bonds that are merely hammered into drilled holes in the rail web. • Unlike exothermic welds, plug bonds, bolted bonds, and compression bonds can be installed in the rain. • Splice bar to rail web bonds may be used to detect a break in the splice bar itself. • Where signal bonds cannot be installed from the field side due to tight areas, such as frogs and switches, a multi-purpose bond can be used by drilling through the rail web. • They facilitate testing because their removal and reinstallation is far easier than cutting and splicing an exothermically welded bond. 10.3 EXTERNAL WIRE AND CABLE 10.3.1 General Various types of cable and methods of installation are required for transit signal systems. Main cables are those cables that run between housings or that contain conductors for more than one system function. Local distribution cables are those cables running between housing and an individual unit of equipment. In selecting the method of installation of external wire and cable, consideration should be given to cost, maintenance, and type of right-of-way. 10.3.2 Trackwork Requirements When determining the location of external wire and cable the following should be considered: • Conduit and cable location • Maintenance of trackwork • Drainage because surface-mounted cable troughs can obstruct the flow of surface water, so provision must be made for storm water to flow around or under the cable trays • Locations of pull boxes, handholes, manholes, duct banks, etc. • Compaction of soil and subballast • Location of cable trough • Visual impact • Staging of construction so that ballast is not fouled with subgrade fines from cable excavation 10.3.3 Types of External Wire and Cable Installations 10.3.3.1 Cable Trough A cable trough system is a modular surface trench that protects and provides continuous accessibility to the signal cables. Signal cables can exit and enter the cable trough system either from the bottom or sides. Trough segments are typically installed in the ballast, often just beyond the ends of the cross ties. Where the cable needs to cross the track, the cable trough is often laid

Track Design Handbook for Light Rail Transit, Second Edition 10-22 in the crib between two cross ties. Cable trough may be in locations where maintenance trucks might drive over it and hence should be capable of supporting an H-20 highway load at any point. The typical cable trough installation requires a trench of minimum width and depth to provide free access to the top of the trough while maintaining crushed stone alongside and below the trough so as to provide free passage of storm water below. Leveling blocks are placed in the bottom of the trench to keep trough segments properly aligned. The maximum stone particle size beneath the trough should not exceed ¾ inch [19 mm]. Fill material should not be placed on frozen ground and should be tamped. The cable trough should be placed so that the uppermost part is 1 inch [25 mm] higher than the surrounding ground or ballast surface. Where cable trough is located within the ballast section, its installation typically occurs after the track construction is completed and the ballast is fully dressed. It is important that the cable trough installer restores the ballast section to its original configuration and does not foul the ballast stone with unsuitable materials. Signal maintainers like cable trough since it provides easy access to the entire length of the cable, making it much easier to troubleshoot faults compared to cables that are either in a duct bank or strung on overhead poles. Track maintainers generally dislike cable trough because it can interfere with track maintenance activities, particularly cross tie replacement and ballast tamping. Where cable trough is located in a ballast crib, it is typically necessary to use manual methods of ballast tamping on the adjacent cross ties. 10.3.3.2 Duct Bank The underground duct system can be completely encased in concrete with a minimum clearance of 2 inches [50 mm] between conduits and a minimum cover of 3 inches [75 mm] to the outside edges of the concrete. If a non-metallic conduit is not encased in concrete, allow 18 inches [460 mm] of separation for signal cables carrying 0 to 600 volts from low voltage cables. For cables carrying over 600 volts, non-shielded cables should be installed in rigid metal conduits with a minimum cover of 6 inches [150 mm]. For cables carrying over 600 volts in rigid non-metallic conduits, the conduit should be encased in no less than 3 inches [75 mm] of concrete or have 18 inches [450 mm] of cover if not encased in concrete. Cables are connected to the duct bank systems using handholes, pull boxes, and manholes for proper pulling points or cable routing. A minimum cover of 30 inches [760 mm] is recommended for protection (per AREMA Communications & Signaling Manual, 10.4.1.F.1 and F.2) when signal cables pass under tracks, ballast, or a roadway. Ideally, all underground ducts will be installed prior to installation of the subballast so that the latter is not disturbed by subsequent excavation. However, this sequence results in conduit stub- ups into the track area that must be protected during placement of the subballast and subsequent track construction activities. It is important that the responsibility for the care of these duct bank risers be assigned in the contract documents. Deferring the construction of the risers until after the track is installed is not a total solution. At a minimum, specifications should require the conduit installer to avoid fouling the ballast and subballast with excavated material and to replace the excavated material with identical granular fill and ballast complying with the original specifications. Regardless of construction sequence, close coordination between designers and between constructors is essential.

Transit Signal Work 10-23 10.3.3.3 Conduit Encased or direct burial conduit should be installed as outlined above or as required by the National Electric Code, Article 300-5 and 1110-4(b). Normally, conduits are installed with not less than 30 inches between the top of the conduit and finished grade. 10.3.3.4 Direct Burial Signal cable and wire should be buried to a uniform depth where practicable, but not less than 30 inches [760 mm] below finished grade. Where signal cable and wire is installed within 10 feet [3 meters] of the centerline of any track, the top of the cable should be a minimum of 30 inches [760 mm] below the subballast grade. Signal cables and wires should be laid loosely in the trench on a sand bed a minimum of 4 inches [100 mm] thick and covered with a minimum of 4 inches [100 mm] of sand before backfilling. Backfill should be compacted to not less than 95% of the maximum dry density of the respective materials, as determined by AASHTO Test Designation T-99, or to the original density of compaction of the area, whichever is greater. Where direct burial signal wires cross the tracks, it is beneficial to install the wiring prior to the tracks. This improves the integrity of the track structure, but complicates signal installation. Any trenching that must be done anywhere in the completed ballast section must avoid fouling the ballast with excavated spoil. Backfilling must use proper ballast and subballast materials and be thoroughly compacted. It often will be necessary to re-tamp the ballast not only over the trench but also for several ties on each side on the disturbance. Signal cables can be plowed in at a depth of 30 inches [760 mm] and 12 inches [300 mm] beyond the toe of subballast. Avoiding the track ballast and subballast is important to maintaining the structural integrity of the track. 10.4 SIGNAL INTERFACE 10.4.1 Signal/Trackwork Interface Signaling and trackwork interface issues include the following: • Location of insulated joints • Location and mounting requirements for impedance bonds, train stops, track transformers, junction boxes, and bootleg risers • Physical connection of impedance bond track cables and track circuit wiring • Location and mounting layout of track switch operating mechanisms and switch machine surface and subsurface areas (ballast, direct fixation, and embedded) • Cable and conduit requirements for interconnection of signal apparatus at track • Location and installation of train stops, inductive loops, transponders, wheel detectors, and axle counters • Interface pick-up with the traffic signal system • Location of block outs and foundations for wayside signal equipment • Electromagnetic interference/electromagnetic compatibility (EMI/EMC) • In cab-signaled territory, coordination of track alignment civil speeds with available cab signal speed commands

Track Design Handbook for Light Rail Transit, Second Edition 10-24 • Grounding • Yard signaling • Grade crossing warning systems • Wayside equipment housings and cases • Corrosion control • Cross tie spacing for signal control equipment, impedance bonds, train stops, and switches • Tie size and length requirements for switch machines and derails • Signal cable connection to rail at special trackwork • Physical connection of switch machines to special trackwork, with adjustment and testing • Loop or transponder mounting on track for train-to-wayside communication • Spaces for cables and conduit passing beneath the rail • Location of guard and restraining rails with respect to insulated rail joints • Horizontal clearance between track and wayside signals and equipment • Vertical clearance between track and signal equipment • Space and drainage for switch machine in direct fixation or embedded track • Provision for installation of snow melters • Location of switch indicators for embedded track • Location of cross bonding and negative return cables • Location of speed limits • Ballast resistance • Foundations for cases, housings, pushbuttons, and signals 10.4.2 Signal-Station Interface The following signal equipment is typically installed at station locations: impedance bonds, inductive loops, bootleg risers, junction boxes, and transponders. If the station is located near an interlocking or highway crossing, there should be sufficient room from the end of the platform to the signal equipment (impedance bonds and signals) and insulated joints if required. This may affect the site layout of the station and might control the position of the platform with respect to the crossing. 10.4.3 Signal-Turnout/Interlocking Interface The following signal equipment can typically be found at turnouts and interlockings: • Switch machines • Impedance bonds • Inductive loops including speed command loops • Train stops • Bootleg risers • Junction boxes • Switch controllers • Electric locks • Transponders • Wire and cables • Signal and power bonding

Transit Signal Work 10-25 • Cases/signal equipment houses (“bungalows”) • Signals • Snow melter systems The design of the track circuit and fouling protection used will determine the location of insulated joints in the special trackwork. Typically in transit applications, the insulated joint should be located approximately 23 to 25 feet [7 to 7.6 meters] ahead of the switch points to allow for the use of plug rails for bonded insulated joints. The size of the turnouts and crossovers determines the speed at which the train can operate. This speed should be one of the available cab speeds. The insulated joint for the turnout fouling must be located with a minimum of clearance, taking into account the longest overhang from the front of the train to the first axle of any equipment that may operate on the track. Location of insulated joints at turnouts and interlockings needs to consider the distance from the signal or point of switch to ensure switch detector locking. This distance can vary from 25 feet to 50 feet. 10.5 CORROSION CONTROL Leakage of stray currents into the ballast bed and earth can be a significant problem if the cables running from the rails are electrically connected to the impedance bond housing case and the case is in contact with the earth. This can occur if the cases are mounted on reinforced concrete where the mounting bolts contact the re-bar, if the bottom of the case is resting on concrete, or if dirt and debris accumulate between the bottom of the case or signal equipment and the concrete. An accumulation of ballast, dirt, or other debris around the locations where the cases or signal equipment are installed along the right-of-way can also provide a path for current leakage. This type of installation can result in a continuous maintenance problem if an effectively high rail-to- earth resistance cannot be achieved. Some impedance bonds are located outside the tracks on timber ties to eliminate points of possible contact with earth. The center taps of the impedance bonds should be insulated from the mounting case. Switch machines also must be electrically isolated from the running rails. Any embedded switch machines and junction boxes need to be insulated from the rail and insulated from concrete or other material. Dissimilar metals shall be isolated or separated for corrosion control. Materials selection needs to consider the environmental conditions where the items are being installed. Yard tracks should be isolated from the main line tracks to reduce corrosion. For additional information on corrosion control, refer to Chapter 8. 10.6 SIGNAL TESTS 10.6.1 Switch Machine Wiring and Adjustment Tests Switch machine wiring and adjustment tests verify the wiring and adjustment of the switch machine. These tests should preferably be carried out, in conjunction with the track installer, to

Track Design Handbook for Light Rail Transit, Second Edition 62-01 confirm throw rod capability, ensure point closure, and ensure proper nesting of the switch point rail to stock rail. 10.6.2 Switch Machine Appurtenance Test Switch machine appurtenance tests verify the integrity of switch machine layout by taking resistance measurements across the following assemblies: • Center insulation of the front rod • Front rod to switch point • No. 1 vertical or horizontal switch rod center insulation • Throw rod insulated from No. 1 switch rod • Point detector piece insulated from switch point • Lock rod insulated from front rod • Other vertical rods as required per layout • Switch machine insulated from the running rails • Switch machine extension plate insulated from the running rails 10.6.3 Insulated Joint Test Insulated joint tests measure the resistance between two ends of the rail separated by insulating material. An insulated joint checker requires the traction power system to be disconnected. Any reading less than 30 ohms should be evaluated. Measurements for a set of insulated joints should be within 30% of each other or they should be rechecked. Insulated rail joint tests for AC track circuits can be performed using a volt-ohm-meter. 10.6.4 Impedance Bonding Resistance Test Impedance bonding resistance tests, using a low-resistance ohm-meter, ensure that a proper connection has been made. 10.6.5 Power and Signal Bonding Test The power and signal bonding test is to ensure that the resistance across the rail connection is not greater than recommended by AREMA Communications & Signaling Manual, Part 8.1.30 and 8.1.31. 10.6.6 Negative Return Bonding Test Negative return bonding tests verify the resistance of each mechanical or welded power bond using a low-resistance ohm-meter.

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TRB’s Transit Cooperative Research Program (TCRP) Report 155: Track Design Handbook for Light Rail Transit, Second Edition provides guidelines and descriptions for the design of various common types of light rail transit (LRT) track.

The track structure types include ballasted track, direct fixation (“ballastless”) track, and embedded track.

The report considers the characteristics and interfaces of vehicle wheels and rail, tracks and wheel gauges, rail sections, alignments, speeds, and track moduli.

The report includes chapters on vehicles, alignment, track structures, track components, special track work, aerial structures/bridges, corrosion control, noise and vibration, signals, traction power, and the integration of LRT track into urban streets.

A PowerPoint presentation describing the entire project is available online.

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