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Monitoring a Light Rail Signaling System: Where Does Safety Critical End, and how should we do this with Free Software

Donald Carr, University of Guadalajara

At the light rail system in Guadalajara, Mexico, there is a desire to always know where the trains are at all times, including seconds early/late for each train, spacing between trains, etc. This information is critical to keeping the trains evenly spaced and giving better service. To improve quality, you must measure the quality. Actually, on line two in Guadalajara, all of the signals that need to be monitored (the track circuit states from the signaling system) in order to derive this information are currently being monitored, and are available in central control. However, the information is only used to illuminate LEDs, visible to operators, on a board in central control, and are not available to be read by a computer. The problems is to feed these values into a data acquisition system where they can then read by a computer and then processed and used to generate the information above. This paper will examine how the signals are processed to generate this information, and describe a free software program that has been written to do just that. The problem is, where does safety critical end, and, what is the appropriate way to let this program evolve and get better, and eventually meet or exceed the standards of proprietary software to do the same thing. Even though this is a much smaller project, I will compare and contrast this with the evolution of the Linux Kernel and other free software projects, and the ad-hoc methods used for free software to, over time, achieve an extremely reliable operating system, and discuss where it is appropriate to use similar "free software" development methods, and when it is not. Finally, we will discuss how this fits in with how new signaling systems are done, and how it can evolve to be the "SCADA" portion of a new signaling / interlocking system, as new systems, we no longer use physical boards with LEDs, the information is displayed on computer screens, and the supervisory outputs are also through the the computer, with no physical switches / buttons.