Track circuits
A track circuit is a simple electric device to detect the presence of trains on the track, which can be used to inform the signalmen, and to control the relevent signals.
Firstly, the track is divided into electrical sections using insulated rail joints.
A DC battery (or AC equivalent) is connected to the rails at one end, with a series resistor.
A relay (or other detector) is connected to the rails at the other end of the section.
With no trains in the section, the electric circuit is complete and the relay energises (picks up) and changes the signal to green.
When a train enters the section, its wheels short-circult the track circuit current, and the relay de-energises (drops), changing any signal to red.
Should a rail or wire break, the relay drops in a fail safe manner.
Track circuits need insulated sleepers, and reasonably high impedence ballast. Steel sleepers can short the rails and make track circuits impractible. Wooden sleepers are fine, except in tropical countries where termites are a problem.
In the UK, wheels with wooden hubs that defeated the track circuit put off the installation of track circuits for many years, until they could be replaced.
To give things some perspective, the battery might generate 3V DC, while the relay might receive 1V DC.
Electrified Railways
Most electrified railways use the running rails to return traction current to the substations. Traction currents of the order of 1000A must co-exist with track circuit currents of 1A.
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Accidents involving track circuits
It is usually the lack of track circuits that cause accidents, though very rarely there are euipment failures.
Accidents caused by lack of track circuits
- Quintinshill
- India - steel sleepers have made track circuits impracticable in many places.
Accidents caused by faulty track circuits
- Cowan rail crash - insulating sand on rail caused train to "disappear".