A track circuit is an electrical device used to prove the absence of a train on rail tracks to signallers and control relevant 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
A railway track or railroad track, also known as a train track or permanent way, is the structure on a railway or railroad consisting of the rails, fasteners, railroad ties and ballast, plus the underlying subgrade. It enables trains to move by providing a dependable surface for their wheels to roll upon. Early tracks were constructed with wooden or cast iron rails, and wooden or stone sleepers; since the 1870s, rails have almost universally been made from steel.
New railway concrete sleeper
Traditional railway track showing ballast, part of sleeper and fixing mechanisms
Track of Singapore LRT
Ballastless high-speed track in China