A 'tripcock' is a piece of safety equipment on a train comprising an arm that protrudes to make physical contact with a 'trainstop' (a piece of lineside equipment set at the same height as the tripcock) as the train passes. If contact is made between a tripcock and a trainstop, the train's brakes are automatically applied. Trainstops are typically located prior to signals and are raised into position when the signal shows danger.
A 'tripcock test indicator' is a sign that can be seen by the trackside of some DC electric railway lines just before a section of track equipped with trainstops. A tripcock test indicator's function is to be triggered by a passing train to test that its tripcocks are set to the correct height to be triggered by a trainstop on the line ahead. When activated, a tripcock test indicator will illuminate with a double 'T' momentarily until being extinguished by a second detector.
During a test, should the sign fail to be illuminated and extinguished in an expected pattern, train crews must assume that the tripcocks are not functioning properly and appropriate action should be taken.