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 relevant signals.

Non electified lines

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.

A small section of rail near the blockjoint is not protected against rail breaks.

Track circuits need insulated sleepers, and reasonably high impedance 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.

Reliability

While in theory it would be enough to insulate only one of the two rails between adjacent track circuits, there is a risk that the battery of track circuit A might have a false feed to the relay of track circuit B. This risk can be reduced by having insulated rail joint on both rails.

The polarity of adjacent track circuits is reversed wherever possible as an additional precaution.

Other dangers

Tracks through damp and muddy sections, such as tunnels, can suffer from potential false feeds caused by the mud acting as a battery.

Such problems can be tackled by Tunnel stick circuits that only allow the track relay to pick up if the train is occupying the next track circuits.

The track relay itself has to be made as fail safe as possible, so for example a surge of current does not weld any contact of the relay with the relay in the up or green position.

Ideally, all track circuits should be indicated to a signal box, so that if they fail, there is some chance that the signalmen will notice sooner of later. The track circuits on Cowan bank affected by sand on the rails were NOT indicated to any signalbox.

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.

Track currents can be dealt with by

• single rail returns, where one rail takes all the traction current, and the other rail is used for the track circuit.

• double rail returns, where both rails carry traction current, split even between the rails by impedance bonds, with the track circuit superimposed.

• on the London Underground there are four rails: the two running rails carry the track circuits, while the 3rd and 4th rail carry the traction voltage, positive and negative respectively.

Jointless track circuit

So-called jointless track circuits use audio frequency tuned circuits to create what amounts to a blockjoint. Typical frequencies are 1700Hz and 2300Hz on one track and 2000Hz and 2600Hz on the other. Jointless track circuits eliminate most of the impedance bonds that electrified railways would otherwise require.

Cab signalling

The track circuit current may be modulated at different frequencies, which can be detected by equipment on the train to provide the driver with a signal indication inside the cab. This is called cab signalling

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
• Hawes Junction train disaster. Signalman forgets about light engines on main line, and express signalled onto it.

• 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".
• Clapham Junction rail crash – involved a track circuit relay, but the real causes were sloppy maintenance work and lack of double switching.

See Also

• Cab signalling