Editing Signal passed at danger (section)

===Train Protection & Warning System===
{{Main article|Train Protection & Warning System}}

On the UK mainline, [[Train Protection & Warning System|TPWS]] consists of an on-board receiver/timer connected to the [[Emergency brake (train)|emergency braking system]] of a train, and radio frequency transmitter loops located on the track. The 'Overspeed Sensor System' pair of loops is located on the approach to the signal, and will activate the train's emergency brake if it approaches faster than the 'trigger speed' when the signal is at ''danger''. The 'Train Stop System' pair of loops is located at the signal, and will activate the emergency brake if the train passes over them at any speed when the signal is at ''danger''.

TPWS has proved to be an effective system in the UK,{{cn|date=May 2017}} and has prevented several significant collisions.{{cn|date=May 2017}} However, its deployment is not universal; only those signals where the risk of collision is considered to be significant are fitted with it.