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77 A.1 Introduction Railroads are unique in the transportation industry in that they operate a substantial portion of their networks with vehicles simultaneously moving in both directions on a single line. To use a highway analogy, this would be like cars moving in opposite directions on a single lane road, with the need to determine how they pass each other when moving in opposite directions and how a faster car can pass a slower car moving in the same direction. A.2 Single Track Operations Railroads solved these problems by constructing passing sidings at strategic locations, generally between 5 and 15 miles apart. When trains moving in opposite directions encounter one another, one of the trains is routed into a passing siding to let the opposing train pass. See Figure A-1, Case 1. The train dispatcher coordinates the movement of trains and uses the signaling system, or in some cases voice communication by radio, to instruct the trains where to go, with the goal of efficiently moving traffic over the network. In such a single track operation, whenever two trains meet one another, one or both will be delayed. One train is routed into a siding, where it will stop and wait for the opposing train. The train moving in the opposite direction may see some delay if the train it is to meet has not pulled completely into the siding when it is approaching the meet location. Generally, the dispatcher will try to minimize the delay by routing the first train to arrive at a meet location into the siding so that the opposing train can pass by without delay, but the timing may not always be perfect. One train can overtake another on a single track, but this will cause significant delay to the remaining traffic, slowing overall movement on the route. Consider a typical single track line with passing sidings located 10 miles apart. As seen in Figure A-1, Case 2, trains moving in the same direction will normally space themselves about two sidings, or 20 miles, apart, as they approach a train moving in the opposite direction. If a dispatcher wants to overtake one train with another, the first train will have to be held at a siding until the following train catches up to it from two sidings back. The higher priority train will then have to get two sidings ahead before the lower priority train can proceed again. All during this time, there can be no movement by trains in the opposite direction, since the two trains running in the same direction will occupy a siding location where opposing trains would normally meet. A P P E N D I X A Discussion of Train Prioritization and Effect on Line Capacity
78 Capacity Modeling Guidebook for Shared-Use Passenger and Freight Rail Operations A.3 Double Track Operations A double track rail line is similar to a two-lane highway. Trains moving on double track lines generally stay to the right, and they typically follow one another at a common speed since it is difficult to overtake a slower train by running on the âleft handâ track. In contrast to single track operations, trains meeting one another on double track can pass freely without delay to either train. Passing sidings, like those used on single track lines, are generally not needed or provided. Railroads provide crossovers at regular intervals on double track to allow trains to move from one track to the other. Like sidings on single track, crossovers are typically spaced about five to 15 miles apart. These crossovers are provided primarily to allow trains to move around a section of track that has been closed for maintenance, or to access industries whose sidings and spurs are located on the opposite side of the tracks. Crossovers are sometimes used to allow trains to Figure A-1. Three illustrative cases of rail line capacity consumption.
Discussion of Train Prioritization and Effect on Line Capacity 79 overtake one another. Crossover speed limits depend on the switch angle, and crossover moves often take place at reduced speed. In describing how one train can overtake another on a double track line, it may be easiest to compare the operation to a two-lane highway. If a car wanted to pass a slower vehicle on a two- lane road, there would have to be a break in the opposing traffic of about a mile before the car could safely pull out into the opposing lane and complete the overtake. It would take perhaps a quarter or a half of a mile to complete the maneuver and pull safely back into the proper lane. It takes significantly more space and time to execute the same maneuver on a railway. The break in opposing traffic has to be in the order of 50 miles to avoid delaying the opposing trains, and the distance to complete the overtake would be in excess of 20 miles. It is possible for one train to pass another in less distance, but this requires that the train being overtaken stop to facilitate the move. Further, any opposing trains must stop to allow one of the trains to run on their track. As an example, consider two trains moving in the same direction, with a slower train, with a capability of achieving 40 mph, running in front of a faster train, with a capability of 60 mph. Normally, the second train will follow the first by about five to seven miles because of the way the signaling system separates the trains. If the train dispatcher wanted to allow the faster train to overtake the slower train, and if the opposing track were clear, the slower train would first be routed onto the opposing track at one of the crossover locations, then the faster train would overtake it on the regular line. The slower train is normally routed through the crossover to the opposite track because any speed restriction it has to obey while passing through the crossover will affect it less. To accomplish the overtake, the faster train first has to catch up to the slower train and pass it completely, then run far enough ahead that the slower train can move in behind it without being further slowed by the signaling system. These distances are shown in Table A-1. Since the difference in speed between the two trains is 20 mph, the complete overtake would require about 40 minutes to accomplish. Since the slow train is progressing at 40 mph during this time, the total distance it occupies the opposite track would be a minimum of about 30 miles. This assumes a crossover is located at the right place to allow the train to move back to its regular track. If a crossover is not handy, the train will have to continue to the next crossover to switch back over. A.4 Train Priorities Class I railroads typically use a system of stated priorities to help ensure that trains move through the rail system in an orderly fashion with usually six or so tiers of priority. In general, passenger trains and certain intermodal trains receive relatively high priority; coal and grain trains (which are typically less time sensitive) are assigned a lower priority. In reality, however, all traffic other than that assigned the very highest priorities tends to operate on a first-come, first-served basis. Table A-1. Overtake distance required. Catch up to slower train 5 miles Overtake slower train 3 miles Run ahead to clear signals 5 miles Total 13 miles
80 Capacity Modeling Guidebook for Shared-Use Passenger and Freight Rail Operations The practice of minimizing preferential treatment for certain trains helps to create fluid- ity throughout the rail system, thereby benefiting trains of all types. When there is moderate or heavy traffic on a route, railroads can generally make the best use of the physical plant by moving trains in order of departure at similar rates of speed. When some trains are expedited, other trains sharing the network will be negatively affected, and system velocity will decrease. Dispatchers simply do not have flexibility to expedite more than a handful of trains over others without creating disruptions on the system. There are a limited number of situations in which it makes sense to take some trains out of order by moving them ahead of others. Passenger trains must move quickly, and intermodal trains generally require an accelerated schedule that is competitive with other modes of trans- portation. Moreover, such trains can be expedited more readily because they have high power- to-weight (horsepower per trailing ton, or hp/ton) ratios and are, therefore, capable of passing other trains quickly. However, the faster that the passenger trains travel, the greater the separation distances needed between them and other trains over the corridor. The capacity footprint of the moving trains results in significant capacity consumption that restricts the flow of other users trying to share the same right-of-way. This verity illustrates a fundamental principle of rail service capacity: The greater is the speed differential between trains, the more quickly is the capacity consumed for that section of track. The most âefficientâ use of a fixed physical rail alignment is for all trains to travel at a common speed. Freight trains, which normally are separated by two siding lengths (or 20 miles) between each other, require double that distance when dealing with prioritized and higher speed passenger trains. As seen in Figure A-1, Case 3, the capacity âwakeâ ahead of a passenger train (on a single track railroad with passing sidings) can be in the order of 40 miles, thus requiring all freight move- ments in the vicinity to be positioned so as not to interfere in any way. Valuable rail capacity is thus consumed whenever such speed differential operations exist combined with high priorities.
