Track circuit

Track circuit

A track circuit is a simple electrical device used to detect the presence or absence of a train on rail tracks, 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 short out an electrical circuit. This circuit is monitored by electrical 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. Each circuit detects a defined section of track, such as a block. These sections are separated by insulated joints, usually in both rails. To prevent one circuit from falsely powering another in the event of insulation failure, the electrical polarity is usually reversed from section to section. Circuits are commonly battery-powered at low voltages (1.5 to 12 V DC) to protect against line power failures. 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.

When no train is present, the relay is energised by the current flowing from the power source through the rails. When a train is present, its axles short (shunt) the rails together; the current to the track relay coil drops, and it is de-energised. Circuits through the relay contacts therefore report whether or not the track is occupied.

Circuits under electrification

In almost all railway electrification schemes one or both of 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 accommodate this, AC track circuits use alternating current signals instead of DC currents. Typically, the AC frequency is in the range of audio frequencies, from 91 Hz up to a 250 Hz. 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. The AC signal can be coded and locomotives equipped with inductive pickups to create a cab signalling system. In this system, impedance bonds are used to connect items which must be electrically connected for electrification purposes but which must remain isolated to track circuit frequencies for the track circuit to function.

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

In some countries, DC track circuits are used on electrified lines. One method provides 5V DC to the rails, one of the rails being the traction return and the other being the signal rail. When a relay is energised and attached to the track, normal voltage is 5V DC. When there is a break in the circuit and there is no train, the voltage rises to 9V DC which provides a very good means for fault finding. This system filters out the voltage induced in the rails from the overhead lines.

Jointless track circuits

Jointless track circuits use audio frequency tuned circuits to create what amounts to a block joint to signalling frequency currents and a very low impedance to electrification power frequency currents.

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. The frequency of modulation is approx 4Hz unless modified on the "MOD" tab, as in the french railway systems.Clarifyme|date=March 2008

TI21 type track circuits use eight nominal frequencies, from 1549 Hz to 2593 Hz. Actual transmission is +/- 17Hz around the nominal frequency. Fact|date=August 2007

Advantages of jointless track circuits:
* Eliminates Insulated Block Joints, a component liable to mechanical failure (both of insulation and by introducing stress to adjoining rails) and maintenance.
* In electrified areas, jointless track circuits require less impedance bonds than any other double rail traction return track circuits.

Disadvantages of jointless track circuits:
* Restrictions on placing impedance bonds, hence any connection for electrification purposes, in or near tuned zones as this may upset the filter properties of the tuned zone.
* Electronic circuits more vulnerable to lightning strikes.


CSEE are another kind of jointless track circuit. It uses 1700Hz and 2300Hz on one track and 2000Hz and 2600Hz on the other. [,%20Test%20and%20Cert.pdf] To reduce the chance of stray currents causing a wrong side failure the basic frequencies are modulate +/15 Hz or so. Different rates of modulation can be detected by eqiuipment on the trains and used for ATC.

Circuit failures

The circuit is designed so that most failures will cause a "track occupied" indication (known as a "Right Side" failure in the UK). For example:

* A broken rail or wire will break the circuit between the power supply and the relay, de-energizing the relay. See exception below.
* A failure in the power supply will de-energize the relay.
* A short across the rails or between adjacent track sections will de-energize the relay.

On the other hand, failure modes which prevent the circuit from detecting trains (known as a "Wrong Side" failure in the UK) are possible. Examples include:

* Mechanical failure of the relay, causing the relay to be stuck in the "track clear" position even when the track is occupied.
* Conditions which partially or completely insulates the wheels from the rail, such as rust, sand, or dry leaves on the rails. This is also known as "poor shunting" ("failure to shunt" in North America).
* Conditions in the trackbed (roadbed) which create stray electrical signals, such as muddy ballast (which can generate a "battery effect") or parasitic electrical currents from nearby power transmission lines.
* Equipment which is not heavy enough to make good electrical contact (shunt failure) or whose wheels must be electrically insulated.
* A rail break between the insulated rail joint and the track circuit feed wiring would not be detected.

Failure modes that result in an incorrect "track clear" signal may allow a train to enter an occupied block, creating the risk of a collision. Wheel scale and short trains may also be a problem. They may also cause the warning systems at a grade crossing to fail to activate. This is why in UK practice, a treadle is also used in the circuitry.

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 susceptible to interference. Speeds may be restricted when and where fallen leaves are an issue. Traffic may be embargoed in order to let equipment pass which does not reliably shunt the rails.

Sabotage is possible; 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. The track circuit therefore did not detect the breaks, and the engineer was not given a stop indication.

Railhead contamination

For a track circuit to operate reliably, the railheads must be kept clean of rust by the regular passage of trains' wheels. Track circuited lines that are not used regularly can become so rusty as to prevent vehicles being detected. Seldom-used points and crossovers and the extremities of terminal platform lines are prone to rusting. Measures to overcome this include:

* Provision of a "depression bar" to detect vehicles;
* Provision of a "stainless steel strip" welded on the railheads;
* The use of a "high voltage impulse" track circuit;
* The use of "axle counters" over the affected section.

Another source of railhead contamination is leaf-fall in Autumn.

Transmission of status

Track circuit occupancy status, along with status of other signal and switch related devices, may be integrated with a local control panel as well as a remote rail control centre. If the track circuit contains a relay, it can be connected to device for sending status information via a communications link. The status can then be displayed and stored for archival for purposes of incident investigation and operations-related analysis. Many signalling systems also have local event recorders for recording track circuit status.

Track circuit clips

A simple piece of safety equipment that can be carried by trains is a track-circuit clip. This is simply a length of wire connecting two metal sprung clips that will clip onto a rail. In case of accident or obstruction a clip applied to a track will indicate that that track is occupied, therefore putting signals to danger. As an example of use, if a train is derailed on a double track, and is foul of the second track, application of a clip to the second track will immediately return signals protecting the second track to danger. This procedure is a much more effective safety measure than attempting to contact a signalling centre by telephone because its effect is immediate and automatic.


The failsafe track circuit was invented in 1872 by William Robinson, an American civil 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.

The first railway signals were manually operated by signal tenders or station agents. When to change the signal aspect was often left to the judgement of the operator. Human error or inattentiveness occasionally resulted in improper signalling and train collisions.

The introduction of the telegraph during the mid-nineteenth century showed that information could be electrically conveyed over considerable distance, spurring the investigation into methodss of electrically controlling railway signals. Although several systems were developed prior to Robinson's, none could automatically respond to train movements.

Robinson first demonstrated a fully automatic railway signalling system in model form in 1870. 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. When no train was within the block no power was applied to the signal, indicating a clear track.

An inherent weakness of this arrangement was that it could fail in an unsafe state. For example, a broken wire in the track circuit would falsely indicate that no train was in the block, even if one was. Recognizing this, Robinson devised the closed loop track circuit described above, and in 1872 [BrMc81, Ph93] , installed it in place of the previous circuit. The result was a fully automatic, failsafe signalling system that was the prototype for subsequent development.

Although a pioneer in the use of signals controlling trains, the United Kingdom was slow to adopt Robinson's design. At the time, many carriages on UK railways had wooden axles and/or wheels with wooden hubs, making them incompatible with track circuits.


Caused by lack of track circuits

Numerous accidents would have been prevented by the provision of track circuits, including:

* Quintinshill rail crash
* Hawes Junction rail crash
* Geurie crossing loop collision - 1963

Caused by track circuit failure

Much rarer are accidents caused when the track circuits themselves fail. For example:
* Cowan rail disaster, which occurred when sand on the rails insulated the wheels from the rails, causing a failure to shunt that allowed a trailing block signal to improperly display a clear aspect, resulting in a rear end collision.

Broken rails

Track circuits can detect some but not all broken rails:

* Weyauwega derailment - undetected broken rail in turnout.

See also

* Wrong-side failure


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