Track Design Handbook for Light Rail Transit, Second Edition (2012) / Chapter Skim
Currently Skimming:
Pages 346-410
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 346... ...
6-i Chapter 6 -- Special Trackwork Table of Contents 6.1 INTRODUCTION 6-1 6.2 DEFINITION OF SPECIAL TRACKWORK 6-1 6.2.1 Basic Special Trackwork Components 6-2 6.2.1.1 Switches 6-2 6.2.1.2 Frogs 6-3 6.2.1.3 Other Turnout Components 6-5 6.2.1.4 Special Trackwork Layouts 6-6 6.2.1.5 Non-Symmetrical Special Trackwork Layouts 6-11 6.3 LOCATION OF TURNOUTS AND CROSSOVERS 6-12 6.3.1 Horizontal Track Geometry Restrictions 6-12 6.3.1.1 Track Geometry in the Vicinity of a Switch 6-12 6.3.1.2 Turnouts on Horizontal Curves 6-13 6.3.1.3 Track Crossings on Curves 6-13 6.3.1.4 Superelevation in Special Trackwork 6-14 6.3.2 Vertical Track Geometry Restrictions 6-14 6.3.3 Track Design Restrictions on Location of Special Trackwork 6-15 6.3.4 Interdisciplinary Restrictions on Location of Special Trackwork 6-15 6.3.4.1 Overhead Contact System (Catenary) Interface 6-16 6.3.4.2 Train Control/Signaling Interface 6-16 6.3.5 Miscellaneous Restrictions on Location of Special Trackwork 6-17 6.3.5.1 Construction Restrictions 6-17 6.3.5.2 Clearance Restrictions 6-17 6.3.5.3 High Volume of Diverging or Converging Movements 6-18 6.3.5.4 Track Stiffness 6-18 6.3.5.5 Noise and Vibration Issues 6-19 6.4 TURNOUT SIZE SELECTION 6-19 6.4.1 Diverging Speed Criteria 6-19 6.4.2 Turnout Size Selection Guidelines 6-24 6.4.3 Sharp Frog Angle/Tight Radius Turnouts 6-25 6.4.4 Equilateral Turnouts 6-27 6.4.5 Curved Frogs 6-27 6.4.6 Slip Switches and Lapped Turnouts 6-29 6.4.7 Track Crossings (Diamonds)
|
From page 347... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-ii 6.5.4.3 AREMA-Style Split Switches in Embedded Track 6-38 6.5.4.4 Design Guidelines for Embedded Switches 6-39 6.5.4.5 Switch Tongue Operation and Control 6-39 6.5.4.6 Embedded Switch Drainage 6-40 6.5.4.7 Embedded Switch Heaters 6-40 6.5.5 Fully Guarded Tee Rail Switch Designs -- Ballasted Track 6-41 6.5.6 Switch Point Detail 6-43 6.6 FROGS 6-44 6.6.1 AREMA Frog Design 6-44 6.6.2 Monoblock Frogs 6-46 6.6.3 Flange-Bearing Frogs 6-47 6.6.3.1 Flangeway Depth 6-48 6.6.3.2 Flangeway Ramping 6-48 6.6.3.3 Flange-Bearing Frog Construction 6-49 6.6.3.4 Speed Considerations at Flange-Bearing Frogs 6-50 6.6.3.5 Wheel/Flange Interface 6-50 6.6.4 Improved Design for Solid and Railbound Manganese Frogs 6-51 6.6.5 Spring and Movable Point Frogs 6-51 6.6.6 Lift Over ("Jump") Frogs 6-51 6.6.7 Frog Running Surface Hardness 6-54 6.7 FROG GUARD RAILS 6-54 6.8 WHEEL TREAD CLEARANCE 6-55 6.9 SWITCH TIES 6-55 6.10 RESTRAINING RAIL FOR GUARDED TRACK 6-56 6.11 PRECURVING/SHOP CURVING OF RAIL 6-56 6.11.1 Shop Curving Rail Horizontally 6-56 6.11.2 Shop Curving Rail Vertically for Special Trackwork 6-60 6.12 LIMITED SOURCES OF SUPPLY FOR SPECIAL TRACKWORK 6-60 6.13 SHOP ASSEMBLY 6-60 6.14 REFERENCES 6-61 List of Figures Figure 6.2.1 Turnout layout 6-3 Figure 6.2.2 Frog on a horse's hoof 6-4 Figure 6.2.3 Frog angle -- North American practice 6-4 Figure 6.2.4 Single crossover (right hand)
|
From page 348... ...
Special Trackwork iii-6 Figure 6.2.7 Single slip switch 9-6 Figure 6.2.8 Double switch lapped turnout -- three frogs 01-6 Figure 6.2.9 Full grand union 01-6 Figure 6.2.10 Double wye [3] 11-6 Figure 6.3.1 Right-hand turnout with a left-hand switch 81-6 Figure 6.4.1 Turnout and crossover data 02-6 Figure 6.4.2 No.
|
From page 349... ...
6-1 CHAPTER 6 -- SPECIAL TRACKWORK 6.1 INTRODUCTION Light rail vehicles, like all steel-flange-wheeled railway equipment, need to be able to transfer from one track to another or to cross intersecting tracks. The fabricated track components and accessories needed to support and direct the rail car at these locations are collectively called special trackwork.
|
From page 350... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-2 • Turnouts and crossovers, including switches, frogs, guard rails, stock rails, and closure rails; rail fastening assemblies unique to turnouts; and miscellaneous components associated with turnouts, including switch rods and gauge plates. Crossover tracks, double crossovers including the central crossing frogs or diamond area, and single and double slip switches are included in this category.
|
From page 351... ...
Special Trackwork 6-3 technically incorrect. In addition, the rest of the English-speaking world uses the word "points" for what North American track designers call a "switch." The stock rails are the running rails immediately alongside of the switch rails against which the switch rails lay when in the closed position.
|
From page 352... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-4 Figure 6.2.2 Frog on a horse's hoof Typically, both rails passing through a frog are straight, although it is possible for one or both rails to be curved. In North America, straight frogs are commonly designated by a number that indicates the ratio of divergence of both rails from a common frog centerline, as illustrated in Figure 6.2.3.
|
From page 353... ...
Special Trackwork 6-5 when using information from sources outside of North America, it is critical to understand the dimensional units being employed. It's not always obvious.
|
From page 354... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-6 devices are positioned at the beginning of the turnout opposite the switch-connecting rods near the point of the switch rails. 6.2.1.4 Special Trackwork Layouts Arrangements of individual turnouts can create a variety of track layouts, thereby permitting many alternative train-operating scenarios beyond the simple divergence offered by a simple turnout: • A single crossover (see Figure 6.2.4)
|
From page 355... ...
Special Trackwork 6-7 one diagonal axis called the "long diagonal" and the "end frogs" will be different from the "center frogs." If the angle of the intersecting tracks is less than that in a No. 6 frog (9o 31' 38")
|
From page 356... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-8 Figure 6.2.5 Double crossover Figure 6.2.6. Single-track and double-track crossings Combinations of turnouts and track crossings are used to produce route junctions.
|
From page 357... ...
6-9 vehicle entering a junction from any direction to exit it on any of the other three legs. Such layouts were common on legacy streetcar systems, but are rarely seen on modern LRT.
|
From page 358... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-10 Figure 6.2.8 Double switch lapped turnout -- three frogs Figure 6.2.9 Full grand union
|
From page 359... ...
Special Trackwork 6-11 Figure 6.2.10 Double wye [3] 6.2.1.5 Non-Symmetrical Special Trackwork Layouts Track alignment engineers, often rightfully, consider their work to be an art and are therefore fond of smooth alignments and symmetry.
|
From page 360... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-12 identical, parallel tangent track grades; otherwise, a vertical curve may be required to compensate. • Double crossovers are usually designed with symmetrical layout along the centerline of the alignment.
|
From page 361... ...
Special Trackwork 6-13 Horizontal curves beyond the heel of the frog should generally be positioned beyond the last long tie of the turnout. In constrained sites, horizontal curves may begin on the long switch ties, but no closer than 20 inches [0.5 meters]
|
From page 362... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-14 one-of-a-kind units and hence very expensive to procure, maintain, and ultimately replace. In addition, depending on the gradients of the intersecting tracks, the curved track may have adverse superelevation.
|
From page 363... ...
Special Trackwork 6-15 one or both tracks may be significantly out of cross-level and that must be considered with respect to operating speed and track twist. 6.3.3 Track Design Restrictions on Location of Special Trackwork While special trackwork can be required in ballasted, direct fixation, and embedded track sections, ballasted track turnouts are generally the most economical to procure and construct.
|
From page 364... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-16 6.3.4.1 Overhead Contact System (Catenary) Interface The installation of the overhead contact wire system (OCS)
|
From page 365... ...
Special Trackwork 6-17 began its movement on a clear traffic signal but which, for whatever reason, had not completed its movement prior to the traffic signal cycling. This is, of course identical to how any traffic intersection functions in the absence of LRT and how in-street junctions are handled on legacy streetcar lines.
|
From page 366... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-18 switch machines above the top of rail are also considered in clearances of trackside obstructions. Refer to Chapter 3 for additional design guidance on special trackwork clearances.
|
From page 367... ...
Special Trackwork 6-19 guideline, this distance can be reduced in areas where modest operating speeds are contemplated. A minimum travel time of 3 to 5 seconds between the special trackwork unit and a more rigid structure is recommended.
|
From page 368... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-20 Figure 6.4.1 Turnout and crossover data
|
From page 369... ...
Special Trackwork 6-21 Figure 6.4.2 No. 6 turnout -- ballasted timber ties with 13' curved switch
|
From page 370... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-22 Figure 6.4.3 No. 8 turnout -- ballasted timber ties with 19'6" curved switch
|
From page 371... ...
Special Trackwork 6-23 Figure 6.4.4 No. 10 turnout -- ballasted timber ties with 19'6" curved switch
|
From page 372... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-24 Limits on operating speeds through the curved side of turnouts are typically based on the turnout geometry and the maximum unbalanced superelevation criteria adopted for the system. In many cases, the closure rail radius zone will impose a greater restriction on operating speed than the switch radius, particularly if tangential switch geometry is not used.
|
From page 373... ...
Special Trackwork 6-25 • Seldom-used crossover tracks that are provided for emergency and maintenance use only should use No. 8 turnouts.
|
From page 374... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-26 straight angle frog. The elimination of the short tangent through the frog will also eliminate the associated lateral jerk and provide a smoother ride.
|
From page 375... ...
Special Trackwork 6-27 6.4.4 Equilateral Turnouts Equilateral turnouts split the frog angle in half between both sides of the turnout, producing two lateral diverging routes. Both sides of the turnout are curved.
|
From page 376... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-28 Figure 6.4.5 Typical curved frog turnout A common yard track layout on streetcar projects is a ladder track that requires double curved frogs with both curves in the same direction, as shown in Figure 6.4.6. The flangeways and track gauges of such layouts must be carefully evaluated using Nytram plots so that wheels are not misdirected at the point of frog.
|
From page 377... ...
Special Trackwork 6-29 6.4.6 Slip Switches and Lapped Turnouts Slip switches and lapped turnouts are often suggested as a means of concentrating a large number of train movements into a constrained site. Such components are very expensive to procure and maintain and are seldom justifiable in a life cycle cost analysis.
|
From page 378... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-30 For an ordinary lateral turnout, both split switch point rails can be straight or one can be straight and the other one curved. Straight switch point rails can be used universally with either right- or left-hand turnouts, but are almost always an inferior choice for a diverging route.
|
From page 379... ...
13-6 rail and possibly lifting the wheel off the top of the switch rail. This raised point rail design is developed by having a lower stock rail seat elevation at each switch plate.
|
From page 380... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-32 point design and must be considered when contemplating the interface between the wheel and switch point. 6.5.3 Tangential Geometry Switches Conventional North American switch points require that wheels make a somewhat abrupt change of direction near the point of switch.
|
From page 381... ...
Special Trackwork 6-33 Figure 6.5.1 60E1A1 (formerly Zu1-60) rail section for a switch point A few North American manufacturers are now producing proprietary tangential geometry switch point rail designs.
|
From page 382... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-34 American street railway operations used tongue switches and mates almost exclusively until very recently. Because the mate provides no wheel guidance, it is generally considered a poor choice for transit lines that contemplate operating low-floor LRVs that have independently rotating wheels.
|
From page 383... ...
6-35 always appreciably smaller wheel diameters. The wide flangeways in the trackwork are also hazardous to pedestrians.
|
From page 384... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-36 wedges will only temporarily correct the problem, and over-tightening can make the switch difficult to throw. Figure 6.5.3 ATEA 75' radius solid manganese tongue switch The ATEA standard tongue switch included a pivoting heel design that could be locked down by lever action.
|
From page 385... ...
Special Trackwork 6-37 system that will be using LRVs with IRWs or might use such articulated car designs in the foreseeable future, should likely specify double tongue switches. However, that choice has significant capital, operations, and maintenance cost ramifications.
|
From page 386... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-38 inferior to the robust design of the cast manganese steel tongue switches and mates, particularly with respect to wear. The number of issues with these designs has diminished with increased experience and the incorporation of better materials.
|
From page 387... ...
Special Trackwork 6-39 Figure 6.5.5 Embedded tee rail switch 6.5.4.4 Design Guidelines for Embedded Switches If pedestrians cannot be reliably excluded from the vicinity of an embedded turnout- -- which is usually the case -- embedded switches should use either traditional North American street railway tongue switches and mates or European design flexive double tongue switches. AREMA tongue switch and mate and double tongue switch designs should not be used, as the flangeway openings are too large for areas where the general public has access.
|
From page 388... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-40 the switch trailable without having to first throw the switch. The most common design, which is still in production, was known as a "Cheatham" switch, after its original manufacturer.
|
From page 389... ...
Special Trackwork 6-41 and the closure of switch tongues or point rails. LRT systems located in places where snow and ice are even occasionally a problem would be well advised to incorporate heaters into each switch.
|
From page 390... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-42 Figure 6.5.6 Fully guarded house top switch Figure 6.5.7 Fully guarded switch with house top and double point of 3 ½ to 5 ½ inches [89 to 140 millimeters]
|
From page 391... ...
Special Trackwork 6-43 A large amount of freeplay between wheel gauge and track gauge is essential for a house top to be an effective guard and to protect an appreciable portion of the curved switch rail. Therefore, house tops are most effective when used with railroad standard wheel gauges.
|
From page 392... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-44 rails with the depressed tread must be custom fabricated, this technique enables the use of offthe-shelf AREMA 5100 detail switch points. However, the depressed tread stock rail results in a rough ride through the switch for both straight and diverging movements.
|
From page 393... ...
Special Trackwork 6-45 • Frogs in primary track can ordinarily be railbound manganese steel (usually abbreviated as RBM) , heavy wall design, generally conforming to details given in the AREMA Portfolio of Trackwork Plans.
|
From page 394... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-46 Figure 6.6.1 Plan view at frog area with 1¾-inch (45-mm) flangeway Figure 6.6.2 Section at ½-inch (15-mm)
|
From page 395... ...
Special Trackwork 6-47 manganese steel, which can be explosive hardened to match the adjoining rail. The completed frog can be installed in track by the thermite welding process, resulting in a structurally continuous track structure.
|
From page 396... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-48 Flange-bearing design can also reduce impact noise that happens when the wheel tread passes over the open flangeway, and some projects have therefore specified flange-bearing specialwork as a noise control measure. This can be effective provided the transit system also has a rigorous program of wheel truing so as to keep wheel flange height as uniform as possible.
|
From page 397... ...
6-49 Figure 6.6.4 Section at ½-inch frog point, flange bearing As a guideline, the ramp ratio should be no steeper than 1 divided by twice the design speed in kilometers per hour. Hence, if the design speed is 30 mph [48 km/h]
|
From page 398... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-50 ramping length on a monoblock frog will be limited to only the central block if the arms are made of tee rail. Flange-bearing frogs tend to develop a wheel wear groove in the floor of the flangeway that can steer the wheels.
|
From page 399... ...
Special Trackwork 6-51 However, some older models of light rail vehicles power both axles from a single motor ("monomotor" truck design)
|
From page 400... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-52 Figure 6.6.5 Contoured welded monoblock frog
|
From page 401... ...
Special Trackwork 6-53 Figure 6.6.6 Lift over, "jump" frog tread support and the overall amount of noise attributable to the light rail system will be reduced. Jump frogs are actually a very old concept, having appeared in the catalogs of trackwork manufacturers from over a century ago.
|
From page 402... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-54 Jump frogs have been adopted by several freight railroads for seldom-used sidetracks from hightonnage freight lines. While transit operations might consider jump frogs so as to reduce impact noise and vibration, the railroads have a different incentive -- extending the service life of an essential frog that is used relatively infrequently on the diverging side.
|
From page 403... ...
Special Trackwork 6-55 Installing an adjustable guard rail in embedded track is difficult; therefore, traditional street railway operations typically installed a section of girder guard rail in lieu of a conventional guard rail. Some contemporary embedded track installations provide a segment of U69 guard rail fastened to chairs in a manner that nominally permits adjustment (provided that the fastenings do not become corroded and unusable)
|
From page 404... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-56 turnout. Hence, a concrete switch tie designed for use at a particular location in a No.
|
From page 405... ...
Special Trackwork 6-57 Figure 6.9.1 No. 6 turnout -- concrete ties with 13' curved switch
|
From page 406... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-58 Figure 6.9.2 No. 8 turnout -- concrete ties with 19'6" curved switch
|
From page 407... ...
Special Trackwork 6-59 Figure 6.9.3 No. 10 turnout -- concrete ties with 19'6" curved switch
|
From page 408... ...
Track Design Handbook for Light Rail Transit, Second Edition 6-60 6.11.2 Shop Curving Rail Vertically for Special Trackwork If a special trackwork unit is within a vertical curve, as often happens when embedded trackwork must conform to existing street geometry, it may be necessary to shop curve rails vertically so that they lay uniformly without kinked joints or welds between contiguous rails. This is particularly true when it is necessary to field weld adjoining rails.
|
From page 409... ...
Special Trackwork 6-61 components fit together as specified and are in accordance with approved shop drawings. The fabricator's schedule should allow adequate time for the inspectors to conduct an unhurried examination, and the assembly should have passed an inspection by the fabricator's quality department prior to the arrival of the owner's inspectors.
|
From page 410... ...
Track Design Handbook for Light Rail Transit, Second Edition 26-6 [2] American Railway Engineering and Maintenance-of-Way Association, Portfolio of Trackwork Plans, 2008 Edition.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.