Track circuit
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A track circuit is an electrical device used to prove the absence of a train on a block of rail tracks to control railway signals. An alternative to track circuits are axle counters.
Illustration of track circuit invented by William Robinson in 1872
Track circuit transformer on the right, new axle counter on the left (Slovenia).
Tuning unit of a ZPW-2000 (Chinese variant of UW71) track circuit and a sign indicating "do not stop at circuit boundary", where loss-of-shunt may occur.
Data pickup unit CSEE; end view
Railway signalling
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Railway signalling, or railroad signaling, is a system used to control the movement of railway traffic. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility i
A Class 66 locomotive (right) is waiting at a red signal while a First Great Western (now Great Western Railway) passenger train (left) crosses its path at a junction.
A block signal and a 130 km/h speed limit at kilometer post 547 in Iisalmi, Finland
Short signal blocks on the Toronto Transit Commission subway system. A train (not visible) has just passed the most distant, leftmost signal, and the two most distant signals are red (stop and stay aspect). The next closest signal is yellow (proceed with caution), and the nearest signal shows green (proceed).
Traditional mechanical signals on display at a railway station in Steinfurt, Germany