What is Signal passed at danger?

A signal passed at danger , known in the United States as a stop signal overrun and in Canada as passing a stop signal, is an event on a railway where a train passes a stop signal without authority. In the United States and Canada, this may be known colloquially as running a red, though this idiom principally refers to automobiles passing red traffic signals.

The name derives from red colour light signals and horizontal semaphore signals in the United Kingdom, which are said to be at danger when they indicate that trains must stop. This terminology is not used in North America where not all red signals indicate stop. In the UK, the alternative description signal passed at red is used where a signal changes to red in front of a train due to either a technical fault or in an emergency, such that the train is unable to stop before passing the signal despite being driven correctly.