Track circuit

A track circuit typically has power applied to each rail and a relay coil wired across them.

When no train is present, the relay is energised by the current flowing from the power source through the rails.

The relays and the power supply are attached to opposite ends of the section to prevent broken rails from electrically isolating part of the track from the circuit.

In some railway electrification schemes, one or both of the running rails are used to carry the return traction current.

There are two common approaches to provide a continuous path for traction current that spans multiple track circuit blocks.

This has the disadvantage of only being able to detect breaks in one rail so the more popular two rail system uses impedance bonds to permit traction current to pass between isolated track circuit blocks while blocking current at track circuit frequencies.

This offers many benefits, except to the signalling system, which no longer has natural breaks in the rail to form block sections.

A DPU avoids the need to change the frequency of a whole series of track circuits in a cascade.

The DPU consists of a tuned coil which detects the presence or absence of current in the adjacent rail and picks up or drops a relay accordingly.

One common brand of high-voltage impulse track (HVIT) circuit is made by Jeumont-Schneider.

At the receiver end an R-C circuit integrates the two pulses, which must be of the correct proportions for the relay to pick up.

In electrified areas, a workaround is needed to allow the traction current, of the order of thousands of amperes, to return to the substation.

As a result, some disc-braked vehicles have "scrubber pads" cleaning the wheels to aid in proper track circuit operation.

In the 1995 Palo Verde derailment, saboteurs electrically connected sections of rail which they had displaced to conceal the breaks in the track they had made.

Another form of sabotage, not intended to cause a train crash but merely to make trains stop and slow down unnecessarily in an effort to sabotage an economy or potential injuries, is to tie a wire between the 2 rails, causing a false obstruction signal.

[5][6] The track circuit relies upon an adequate electrical contact between the rail and the wheel; contamination can insulate the one from the other.

A common problem is fallen leaves, though there have been cases where crushed insects have also caused detection failures.

Measures to overcome this include: Insulated blockjoints can be bridged by wheel scale in some circumstances causing one or two track circuits to fail.

This problem can be overcome by introducing a time delay of say 1–2 seconds into the departing track circuit.

This can be done in one of two ways: A simple piece of safety equipment which is carried by all heavy rail trains in Britain is a track-circuit operating clip (TCOC).

In the event of accident or obstruction a clip applied to both rails will indicate that that line is occupied, putting the signal for that section to danger.

TCOCs are ineffective where train detection is not by means of track circuits, such as axle counters or treadles.

[10] The failsafe track circuit was invented in 1872 by William Robinson, an American electrical and mechanical engineer.

His introduction of a trustworthy method of block occupancy detection was key to the development of the automatic signalling systems now in nearly universal use.

A full-sized version was subsequently installed on the Philadelphia and Erie Railroad at Ludlow, Pennsylvania (aka Kinzua, PA), where it proved to be practical.

His design consisted of electrically operated discs located atop small trackside signal huts, and was based on an open track circuit.

For example, a broken wire in the track circuit would falsely indicate that no train was in the block, even if one was.