Innovations in Dynamic Simulation Modeling for Rail and Transit

author list icon

Railroad simulation modeling of signal train control, specifically the accurate representation of train control systems and Positive Train Control (PTC), is discussed using the Northeast Corridor and the Long Island Rail Road as examples.

Computer simulation is a particularly important and useful tool for evaluating different railroad improvement strategies. In general, the more modeling done up front, the less expensive the overall project will be, since modeling enables the plan to be refined to its most essential elements.

The first step in using computer models in railroad planning is to build and calibrate the base case model. This should accurately replicate railroad operations with the existing infrastructure, rolling stock, and schedules. Models typically represent only a portion of a network and they simplify features that are not relevant to the specific purpose of a project. The proper assumptions have to be made about degree of reduction of insignificant features and their applicability to support project goals. It is critical that all simulation results be carefully reviewed and discussed with those familiar with operations.

While computer simulation is an excellent tool for analysis and planning and it is recommended completing as much modeling as possible before starting a railroad improvement program, simulations have some limitations. Large models take longer to develop, they are more difficult to manage and it takes longer to obtain and process results.

Generally, rail and transit simulations have three major types of input: rail infrastructure, signal train control, and schedules. This article discusses only one component of railroad simulation modeling input – signal train control, specifically the accurate representation of train control systems and Positive Train Control (PTC). Two of the most recent models will be used as examples: the models of the Northeast Corridor1 and the Long Island Rail Road. The Northeast Corridor between Washington, DC and New York is a mostly four track rail line with over 50 interlockings2 and is used by Amtrak regional service, Amtrak intercity, the high speed Acela express and several long distance trains, as well as New Jersey Transit and SEPTA services. These trains have different performance and operating characteristics and therefore the train control system is extremely complex.
Figure 1 – West Side Rail Yard for LIRR Trains, Looking East to Penn Station.

The Long Island Rail Road (LIRR) is the busiest commuter railroad in North America, bringing commuters from various locations in Long Island, NY to New York Penn Station (see Figure 1). About 800 trains are operated to New York City on a weekday, with over 40 trains per hour in a rush hour. The train control system is complex and fine-tuned to deliver such volume of trains. It is critical to accurately simulate its operation to identify the impact of Positive Train Control implementation.

Signaling and Train Control Systems

Rail Traffic Controller (RTC) is a commercially available software package by Berkeley Simulation Software (BSS). The WSP | Parsons Brinckerhoff simulation modeling team has a long productive business relationship with BSS and the development team.

Originally RTC didn’t have signaling logic. Trains in the model were spaced based on safe braking distance. With time it became clear that in order to accurately assess the railroad capacity it was necessary to enhance RTC with signaling simulation capability. WSP | Parsons Brinckerhoff provided expertise to the programmers of BSS that allowed for the development of this important part of RTC simulation modeling software. We tested functionality of the new module using the NYC Subway’s No. 7 Line simulation model. Now this program is a powerful railroad simulation modeling tool that accurately represents the characteristics of rail and train control infrastructure, realistically simulates train movements over a variety of rail networks with different levels of complexity, multiple tracks and/or routes, and variable stopping patterns and can be used to simulate complex signaling systems.

The Northeast Corridor’s automatic signaling system is the most complex in the US. It supports high density and high speed train operation with different operating characteristics (e.g., NJ Transit, SEPTA, Amtrak Intercity, Amtrak Regional, and Amtrak Acela) and prior to this project it had never been modeled using the RTC simulation program. To realistically represent train movement and performance of cab signaling, the software required development of new functionalities.

Figure 2 – Railway signals have aspects and indications.

Railway Signals

In the eastern US, railroads operate under the NORAC (Northeast Operating Rules Advisory Committee) rulebook. Railway signals have aspects and indications (see Figure 2).

The aspect is the visual appearance of the signal; the indication is the meaning. Every signal aspect3 of the train control system is described by defining a train speed when passing a signal, within a signal block, and at a next signal ahead. When passing a signal, the position of a locomotive or a first car is important. Some aspects allow for the speed reduction to begin after a first car or a locomotive passes a signal, while others require reducing the speed in advance and not exceeding the governing speed when passing a signal. When coding these features into the model, it is important to coordinate a signal aspect definition so that the exiting block speed of a signal aspect matches an entering block speed of the following aspect.

Very often a signal design dictates that a train should travel with the same reduced speed over a number of consecutive signal blocks without reaching maximum track speed, which means that the same signal aspect should be displayed over this number of signal blocks. A solution had to be developed for such situations because simulation software considered this data input as an infinite loop.

When taking a diverging route, a train reduces its speed to a signal indication that is installed before a turnout (railroad switch). The speed that the signal indicates is defined by the switch geometry. Then, after rear wheels clear an opposing home signal, a train increases its speed to the maximum allowable, even if there is no signal allowing a speed increase in a train-moving direction. The modeling software had to be enhanced to look not only at a train position relative to other trains and signal indications in the same direction, but also at an interlocking signal in the opposite direction so that train speed is accurately represented.

Signal Timing

Sometimes signal design uses timers. A signal timer is a feature that allows for a train to move at a higher speed for a specific number of seconds and, after this period elapses, reduces the train speed to the signal indication. If trains in the network are of the same type, consist4, and have the same operating characteristics, then we would calculate a distance that a train would travel under a higher speed and input a signal duplicator at which a speed should be reduced. If a rail network is heterogeneous, then this distance would be different for each train type. In situations like this a question arises: Do we indeed achieve higher accuracy of model performance by this detailed and time consuming input?

When developing a complex model for a first time, it’s difficult to predict which features of a system are more important than the others, which of them could be represented in the reduced capacity, and what degree of reduction is acceptable. As we started the study, together with the client, a decision was made to reduce representation of timers. This would signify more conservative conditions. Other features were fully represented in the model by additional input or development of new software functionality. Currently we are in the process of developing a signal control line output. The software will automatically draw the control lines as they are input to the model. This will significantly reduce the model quality review time. The timer feature is planned to be developed next.

Positive Train Control

The US Rail Safety Improvement Act of 2008 (RSIA) mandated that Positive Train Control (PTC) be implemented across a significant portion of the nation's rail industry. PTC refers to communication-based/processor-based train control technology designed to prevent train-to-train collisions, overspeed derailments, incursions into established work zone limits, and the movement of a train through a main line switch in the improper position. If an unsafe train movement occurs, PTC will audibly alert the locomotive engineer and display a safe braking distance based on the train’s speed, length, width, and weight, and the grade and curve of the track. If the locomotive engineer does not respond to the audible warning and screen display, the onboard computer will activate the brakes and safely bring the train to a stop.

It is anticipated that the locomotive engineers will operate trains with extra caution to avoid the brake activation. To simulate these processes, the assumptions will have to be made regarding operator precaution. There are speculations that for the first two weeks after PTC implementation, the train engineers will operate trains with extra caution. After a two week period, they will familiarize themselves with the system enough to exercise better judgment about the manner of operation. Also, the software will have to distinguish the reason for brake application. If it’s a normal service speed reduction, then a normal service brake rate should be applied. If it’s a PTC related speed reduction, then PTC enforced braking algorithm will be governing the process of braking a train.

Simulation modeling is an excellent tool to quantify and measure the effect of PTC implementation on system operation. New software functionality is currently under development that will allow for a user input of PTC braking rates, which in turn will govern PTC related brake application. This software development will be used and tested in WSP | Parsons Brinckerhoff’s recent win - the effect of PTC implementation on Long Island Rail Road operation.


Innovation is defined as the act or process of introducing a new idea or method(s). Sometimes it’s a different application or approach that helps to meet new requirements. Every project that uses simulation modeling to support operation analysis is a research project in which new methods are explored.

This article presents just a few of the innovative ideas that  contributed to major software development and enhancement. Making simulation modeling software “smarter”, more intuitive, user friendlier, and with more flexible reporting capabilities contributed to many aspects of our work. It enabled members of the WSP | Parsons Brinckerhoff simulation modeling team to become more versatile in the use and application of simulation modeling software.; We have become experts in the understanding and the utilization of the features and functionalities that we participated in designing, raising the quality of our deliverables and promoting the value we deliver to our clients. Innovations portrayed in this article will be used in rail line and terminal capacity analysis, rail and transit network extension and expansion projects, and quantitative comparison of capacity improvement projects.   

1The Northeast Corridor is an electrified railway line in the Northeast megalopolis of the United States. Owned primarily by Amtrak, it runs from Boston through New York City, Philadelphia, and Baltimore to Washington, D.C.

2In railway signaling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. An interlocking is designed so that it is impossible to display a signal to proceed unless the route to be used is proven safe.

3Different railroads historically assigned different meanings to the same aspect, so it is common as a result of mergers to find that different divisions of a modern railroad may have different rules governing the interpretation of signal aspects.

4Consist - The group of rail vehicles making up a train, or more commonly a group of locomotives connected together for multiple-unit (MU) operation.

back to top