block instrument
noun1
Rail transport, Railways, Transport UK An instrument installed in a cabinet, used in block signalling installations.
- The line is signalled with upper quadrant semaphores and Sykes lock and block three-wire two-position block instruments. [...] The Inspecting Officer concludes that both signalmen must share responsibility for the accident by the irregular release of the block instrument, but remarks that the two-position Sykes instruments, although ample for maintaining safety under normal working, do not show clearly the state of the block section ahead.1960 March, “Talking of Trains: Collision at Slade Green”, in Trains Illustrated, page 133: