How Railway Signalling Works
On 28 October 2026 a high-speed train ran into the back of another that had stopped near Urda, on Spain's Madrid–Seville line; regional authorities said at least 68 people were killed, and an independent commission is investigating. This film explains the fundamentals the investigation will turn on: why steel wheels on steel rails need kilometres to stop; how time-interval working failed at Clayton Tunnel in 1861 and the block system replaced it; how William Robinson's track circuit of 1872 detects trains and fails safe; how interlocking and the Armagh disaster of 1889 shaped the rules; why red, yellow and green signals are spaced by braking distance; how cab signalling such as Germany's LZB and Japan's automatic train control took over at high speed; what the Santiago de Compostela derailment of 2013 showed about gaps in protection; and how Europe's ETCS and moving block work.
One train, one empty track
How railway signalling keeps trains apart.
Too heavy to stop
Steel on steel grips very little.
Time and the telegraph
A stopwatch cannot see a train.
Failing safe
A fault must look like a train.
Red, yellow, green
Each signal watches the block ahead.
Signals in the cab
The line talks to the train.
Where protection ends
A system guards only the track it covers.
Moving block
A safe zone that travels with the train.
The track ahead must be known to be clear
Every signal answers one question.
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