Whenever TU/e computer scientist Mark Bouwman is on the train - which is quite often, he deliberately does not own a car - his attention is almost automatically drawn toward the trackside. After years of research, the various boxes along the railroad tracks hold few secrets for him. As part of a research consortium with the University of Twente and railroad companies ProRail and Deutsche Bahn, Bouwman explored how to safely switch to a new communication system for controlling signals and switches. On Monday, October 23, he will defend his doctoral research at the Department of Mathematics and Computer Science.

Red signal

When you see a signal turn red, it is controlled by a central computer - the interlocking in technical terms - via a direct cable, Bouwman explains. “The interlocking ensures that trains don’t collide. Every European station has its own interlocking that uses the same basic control principle. But nevertheless, they’re all slightly different. And that’s what European railroad companies want to change. In a few years, all field elements within Europe must be controlled using the same protocol.”

That is why they are switching to a different method of communication. No longer via direct cable, but through a fiber optic network. For example, the interlocking will be able to address packets of information to a specific signal: you turn red now. There are several advantages to having such a uniform system throughout Europe, says Bouwman. “It means a sudden expansion of the equipment market. A Dutch signal can then also be used in other countries, and we can use foreign switches. This way, innovations get to the market faster. And by using a fiber optic network, the whole system is less susceptible to cable breaks. Now, if a cable breaks during excavation work, the result is a signal malfunction, which causes all sorts of problems. A fiber optic network is much more flexible so that signals can be rerouted more easily. Separating the interlocking and wiring is also beneficial for durability since the whole system does not need to be replaced every time.”

Stone in the switch

So there are many advantages. But before a new communication standard can be implemented, safety must be guaranteed down to the smallest detail. And that is where Bouwman’s mathematics came into play. “By logically specifying a protocol you check all the possibilities so you know exactly how it works and can lock down the system securely.” For this step-by-step approach, Bouwman had a whole toolkit at his disposal, including the modeling language mCLR2 that was developed at TU/e. “That reprogramming is very important. If you write something down in regular language, this can lead to ambiguities, whereas everything should be as unambiguous as possible. After that, we could start calculating all the possible behaviors of the system. A switch can change position at the right time, but there could also be a stone caught in between, or someone could jam it with a crowbar. It’s important to not make assumptions, but to describe all the states it could potentially be in.”

Supercomputer

In order to efficiently convert specifications to mCLR2 language, Bouwman set up a new system. This was quite a challenge and required a lot of computing power, he explains. “You have to calculate millions, or rather billions of states; all the paths the system could ever take. Within our research group, we use the Mastodont, a supercomputer capable of performing all those complex calculations. But because the state space - all the possibilities and paths in between - had grown so large, even the Mastodont couldn’t handle it.”