The late trains you see – and those you don't
Summer series. Students' projects (4/9) – For her semester project, civil engineering student Chloé Lafaye designed and ran a series of simulations to model how train delays propagate down a rail line and compare the two generations of train signaling systems used in Switzerland.
Frustrated passengers waiting for a late train actually just know part of the story. What they don’t realize is that their late train is just the tip of the iceberg – it’s probably causing all the trains behind it to be late too, since they will have to wait on the track until the first train goes through. This cascading effect, the result of traffic safety requirements, poses a real problem for railway operators.
There are two main types of signaling systems in use today: lateral signaling, the conventional system whereby red-
yellow-green traffic lights are located at regular intervals along the rail line; and what’s known as the Level 2 European Train Control System, which is a new-generation technology that uses onboard electronics and radio-frequency waves to monitor the train’s speed and display signals directly on the train’s console. This second type of system – already in use on some sections of Swiss railways – is more accurate but requires special equipment to be installed on trains. The two systems are not compatible with each other.
Chloé Lafaye, a Master’s student in civil engineering at EPFL, analyzed the effects of cascading delays under each type of signaling system. Her work was overseen by Daniel Emery, a lecturer and researcher at EPFL’s Transportation and Mobility Laboratory, and involved running hundreds of simulations. “If one train is held up coming into a station – because the line is blocked or because a previous train hasn’t yet moved past a signal box, for example – that will hold up all the trains behind it. My model can calculate the waiting time between trains under different signaling systems. I also compared the two generations of systems.”
Lafaye first looked at lateral signaling systems. For safety reasons, trains on the same line must remain an appropriate distance from each other. In lateral signaling systems, this distance is maintained using traffic lights that have been placed at regular intervals. A train sitting between two traffic lights cannot move forward until the train ahead of it has gotten a green light and moved on. That means the trains behind it also have to wait – which can cumulate and result in considerable delays. Lafaye calculated that if the first train is held up by eight minutes, the five trains behind it will have a cumulative delay of 27 minutes and 34 seconds. It isn’t until the seventh train that things will return to normal.
Lafaye also performed her calculation using the new-generation signaling system, where the signals are displayed directly on a train’s console. The signals are based on data transmitted continuously from a control center via RF waves. No external traffic lights are needed – all the information required to operate the train is collected and displayed right on board. Going back to Lafaye’s example of a train that is eight minutes late, with this type of system only the following four trains would also be held up, with a cumulative delay of 23 minutes and 10 seconds. Onboard signals enable trains to travel closer to each other safely and thereby shorten waiting times.
“In the future, technology will enable trains to talk to each other, shortening delays even further,” says Alexandre Barrat, a train conductor for the Swiss Federal Railways.