Reverser
A mechanical treadle that puts a signal to 'stop' can be replaced by a short track circuit and a reverser. A reverser is an electrically engaged latch that allows the signal to be reversed, i.e. placed to green. When the track circuit past the signal is occupied, power to the latch is removed, and the signal reverts to 'stop', red.
Reversers have been used in New South Wales especially to put starting signals to stop. When the signal was put back to stop, the system automatically sent the train on line bell signal (two bells) to the station in advance. This equipment would have helped prevent the Hawes Junction rail crash.
Read more about this topic: Treadle (railway)