tripcock

plural tripcocks

1. rail transportA train-mounted safety device which interacts with a trackside device to stop a train automatically if it passes a signal at danger (showing a red light).

   Chiltern Class 165s are fitted with tripcocks to allow them to operate on LU infrastructure between Harrow-on-the-Hill and Amersham.

   2020 July 1, Richard Clinnick, “Network News: '165' runs through points and into path of LU service”, in Rail, page 20

   1 more example

   When a train runs onto the track, thus connecting the two rails, a short circuit is created, the current drops out and the signal switched to red. The system operates in conjunction with a lever on the tracks. If a train should pass a red light, the lever, which is called a tripcock, comes into play. The tripcock releases the air from the train's air-braking system, thus causing the brakes to come on and stopping the train.

   2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, →ISBN, pages 136–137