signalling

Signalling is the collection of fixed installations, devices, and procedures that control train movement and prevent collisions on a railway line. It includes the physical signals themselves (semaphores, colour-lights, or signals mounted on gantries), the electrical circuits and relays that operate them, the track circuits that detect train presence, and the interlocking equipment that governs which signal aspects can display at any given moment. A signal tells a driver what route is set ahead and what speed they may safely proceed at.

Track circuits form the sensory backbone of most signalling systems. They work by sending a low-voltage current through one rail, across the wheels and axles of any train standing on the section, and back through the other rail. The presence of a train shunts this current, which causes relays to drop and triggers the signals protecting that section to turn red. When the train leaves, current flows freely again, the relays pick up, and the signals return to proceed. This detection happens automatically with no human intervention on the train itself.

Interlocking and mechanical safety

Interlocking equipment is the logic machine of signalling. It locks points (switches) in the correct position when a signal is cleared for a particular route, and it prevents conflicting signal aspects from displaying simultaneously. Traditional mechanical interlocking uses frames of steel bars and levers; modern systems use solid-state electronics or programmable logic controllers. The frame physically forbids the lever for signal A from moving to clear while lever B (which would send trains on a collision course) remains pulled. This mechanical or electrical interlock cannot be overridden by fatigue, carelessness, or pressure.

Signalling systems vary by line density and traffic pattern. Main lines with frequent express traffic typically use multi-aspect colour-light signals mounted on posts or gantries, often working with Automatic Train Protection (ATP) systems that can apply brakes if a driver passes a red signal. Rural branch lines may use lower-cost semaphore signals or even token systems, where the driver must physically carry a token that proves they own exclusive right to a particular section. Yards and sidings use simpler ground signals and hand signals.

The term signalling (British spelling) or signaling (American) originally referred just to the signal hardware itself, but evolved to encompass the entire operating system because no signal makes sense in isolation. A single signal is useless without track circuits to feed it information, interlocking to enforce safe logic, and trained staff to maintain it. Design, maintenance, and replacement of signalling systems demands expertise in electrical engineering, rail-traffic planning, and safety-critical systems.