Accident
One early signal was the "Automatic" signal invented by CF Whitworth. Far from being "automatic" in operation, this was merely a signal that was operated by the signalman but returned to 'danger' once the train had passed, by means of a treadle. There was one of these at each end of Clayton Tunnel, just north of Brighton, and it was the failure of the signal to return to danger during a busy period that led to the worst ever accident on that railway. Because the signal had failed to return to stop with another train approaching close behind, the signalman was forced to wave a red flag at the oncoming train without knowing if the train would see this. After wrongly assuming the train had then left the tunnel, the signalman allowed a further train to enter the tunnel, where it collided with the first which had started reversing.
The biggest flaw in the Whitworth automatic signal is probably that it had no redundancy, and a single stone might jam it. On the other hand, without a treadle, the signalman is more likely to get distracted and forget to put the signal to stop.
Read more about this topic: Treadle (railway)
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