Robust transportation requires reliable and proven solutions. The development of the safety systems is a long-lasting process composed of various phases. They may span weeks or months and focus on different layers of the system.

The realization of specific functionalities in complex environments is dominant feature of the railway signaling systems, which must work according to the design principles and follow strict requirements to achieve the necessary system maturity.

The crucial step in the development process and verification of such systems is testing.
Why is testing important?
In June 2009, the Washington Metro train collision occurred due to a track circuit malfunction caused by ‘parasitic oscillation,’ a peculiar type of oscillation that makes logical systems transmit unreliable and incorrect output signals.

Due to the transmission of ambiguous signals, the system was unable to reliably identify the track occupation by a train. The train which stopped on a faulty track, was not detected by the trackside safety system.

Consequently, the automatic train control system issues a permissive signal for a train behind stopped one to enter the already occupied section at a speed of 89 kph. Despite emergency braking triggered, the driver could not stop the train before reaching the one already in the section.

This tragic event led to nine fatalities and eighty injuries.
The investigation performed by the National Transportation Safety Board showed that both circuits kept switching between showing the track as empty and then occupied almost all the time from June 17, followed by the track circuit part replacement, until the crash.

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