Track circuit

A track circuit is a simple electrical device used to detect the presence or absence of a train on a railroad track, used to inform signallers and control relevant signals.

Principles and operation

The basic principle behind the track circuit lies in the connection of the two rails by the wheels and axle of locomotives and rolling stock to complete an electrical circuit. This circuit is monitored by electronic equipment to detect the presence or absence of the trains. Since this is a safety appliance, fail-safe operation is crucial; therefore the circuit is designed to indicate the presence of a train when failures occur. On the other hand, false occupancy readings are disruptive to railroad operations and are to be minimized.

Track circuits allow railway signalling systems to operate semi-automatically, by displaying signals for trains to slow down or stop in the presence of occupied track ahead of them. They help prevent dispatchers and operators from causing accidents, both by informing them of track occupancy and by preventing signals from displaying unsafe indications.

The basic 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. When a train is present its axles short the track circuit and the track relay coil is de-energised. Circuits through the relay contacts therefore report whether or not the track is occupied.

Each circuit protects a certain length of track, such as a block. These sections are separated by insulated joints in both rails. To prevent one circuit from falsely powering another, the electrical polarity is reversed from section to section. These circuits are commonly battery-powered at low voltages (3 V DC). The relays and the power supply are attached to opposite ends of the section in order to prevent broken rails from electrically isolating part of the track from the circuit.

Circuits under electrification

In almost all railway electrification schemes the rails are used to carry the return current. This prevents use of the basic DC track circuit because the substantial traction currents overwhelm the very small track signal currents.

To accomodate this, AC track circuits use alternating current signals instead of DC currents. The relays are arranged to detect the selected frequency and to ignore DC and AC traction frequency signals. Again, fail safe principles dictate that the relay interprets the presence of the signal as unoccupied track, whereas a lack of a signal indicates the presence of a train.

In this system, impedance bonds are used to connect items which must be electrically connected but which must remain isolated for the track circuit to function.

AC circuits are sometimes used in areas where conditions introduce stray currents which interfere with DC track circuits.

Jointless track circuits

Jointless track circuits use audio frequency tuned circuits to create what amounts to a blockjoint.

Frequencies of the Aster SF 15 type track circuit are 1700 Hz and 2300 Hz on one track and 2000 Hz and 2600 Hz on the other. These frequencies are modulated by a small frequency.

TI21 type track circuits use the following frequencies;

A 1699 Hz Down line B 2296 Hz Down line C 1996 Hz Up line D 2593 Hz Up line E 1549 Hz Down line F 2146 Hz Down line G 1848 Hz Up line H 2445 Hz Up line

A to D are used in two-track areas, while E to H are additional frequencies for use in four-track areas.

Jointless track circuits eliminate most of the impedance bonds that electrified railways would otherwise require.

Circuit failures

The circuit is designed so that most failures will cause a "track occupied" indication. For example:

A broken rail or wire will break the circuit between the power supply and the relay, deenergizing the relay.
A failure in the power supply will deenergize the relay.
A short across the rails or between adjacent track sections will deenergize the relay.

On the other hand, failure modes which prevent the circuit from detecting trains are possible. Examples include:

Mechanical failure of the relay
Material which insulates the wheels from the rail (e.g. sand or dry leaves)
Conditions in the trackbed which create stray electrical signals
Equipment which is not heavy enough to make good electrical contact or whose wheels must be electrically insulated

Different means are used to respond to these types of failures. For example, the relays are designed to a very high level of reliability. In areas with electrical problems different types of track circuits may be used which are less suceptible to interference. Speeds may be restricted when and where fallen leaves are an issue. Traffic may be embargoed in order to let equipment pass.

Sabotage is of course possible. In the 1995 Palo Verde derailment saboteurs connected sections of rail which they had displaced in order to cover up the breaks in the track they had made. The track circuit therefore did not detect the breaks and the engineer was given no indication to stop.

History

The track circuit was invented by William Robinson, making possible the automatic block signal. Robinson's block signal was first demonstrated in model form in 1870 and subsequently installed on the Philadelphia and Erie Railroad at Kinzua, Pennsylvania. It consisted of electrically-operated discs located on top of small signal huts, and was based on an open track circuit. He improved on this with the closed track circuit, replacing the earlier installation in 1872 Ph93.

The "open" operation of the first track circuit installation meant that the short circuit made by the train powered the signal to stop. This was not fail-safe, and was soon changed.

The United Kingdom was rather slow in adopting track circuits, partly perhaps because they were an American invention, and partly because many carriages had wheels with wooden hubs, which would not operate track circuits.