Signal passed at danger

On the other hand, some SPADs involve the driver being unaware they have passed a signal at danger and continue until notified by network controllers, or a collision occurs, as in the Ladbroke Grove rail crash.

Therefore ATP can determine when brakes should be applied in order to stop the train before passing a signal at danger.

Whilst the ideal safety system would prevent a SPAD from occurring, most equipment in current use does not stop the train before it has passed the Danger signal.

On the UK mainline, AWS consists of an on-board receiver/timer connected to the emergency braking system of a train, and magnets located in the center of the track.

A visual indication remains set to remind the driver that they have passed a restrictive signal aspect.

On the UK mainline, TPWS consists of an on-board receiver/timer connected to the emergency braking system of a train, and radio frequency transmitter loops located on the track.

Prior to the introduction of TPWS in the UK, "SPAD indicators" were introduced at 'high risk' locations (for example: the entry to a single track section of line).

Whenever a SPAD indicator activates, all drivers who observe it are required to stop immediately, even if they can see that the signal pertaining to their own train is showing a proceed aspect.

They are designed to fail safe, so that when problems occur, the affected signal indicates danger (an example where this did not happen, known as a wrong-side failure, was the Clapham Junction rail crash due primarily to faulty wiring).

To keep the network running, safety rules enable trains to pass signals that cannot be cleared to a proceed aspect.

If the train reaches the next signal without finding an obstruction, they must obey its aspect, at which point they can revert to normal working.

Failure to do this has caused the following collisions: Except where permissive working is in use, interlocking usually prevents a train from being signalled into a section that is already occupied.