A track circuit is an electrical device used to prove the absence of a train on a block of rail tracks to control railway signals. An alternative to track circuits are axle counters.
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 an electrical circuit. This circuit is monitored by electrical equipment to detect the absence of the trains. Since this is a safety appliance, fail-safe operation is crucial. Hence, the circuit is designed to indicate the presence of a train when failures occur. On the other hand, false occupancy readings are disruptive to railway operations and must be minimized.
Track circuits allow railway signalling systems to operate semi-automatically, by displaying signals for trains to slow 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.
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 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.
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 one section to an adjacent section. Circuits are powered at low voltages (1.5 to 12 V DC). 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. A series resistor limits the current when the track circuit is short-circuited.
Circuits under electrification In some railway electrification schemes, one or both of the running rails are used to carry the return traction current. This precludes use of a basic direct current (DC) track circuit because the substantial traction currents overwhelm the very small track circuit currents. DC can also not be used on track circuits for unelectrified tracks near DC electrified tracks. Moreover, track circuits on tracks that are electrified with alternating current (AC) cannot use current that oscillates at multiples of the traction power frequency to avoid harmonics.
Consequently, track circuits typically use AC in the range of audio frequencies, from 91 Hz up to 10 kHz. The relays are arranged to detect the selected frequency and to ignore DC and AC traction frequency signals. Fail-safe principles dictate that the system interpret the presence of the signal as unoccupied track and a lack of a signal as the presence of a train. The AC signal can be coded and locomotives equipped with inductive pickups to create a cab signalling system.
There are two common approaches to provide a continuous path for traction current that spans multiple track circuit blocks. The simplest method installs insulated track circuit joints on only one of the two rails with the second being a path for the return current and a ground for the track circuit rail. 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.
AC circuits are sometimes used in areas where conditions introduce stray currents, which interfere with DC track circuits.
Jointless track circuits
Modern track is often continuously welded, the joints being welded during installation. This offers many benefits, except to the signalling system, which no longer has natural breaks in the rail to form block sections. The only method to form discrete blocks in this scenario is to use different audio frequencies (AF) in each block section. To prevent the audio signal from one section passing into an adjacent section, pairs of tuned circuits are connected across the rails at the section boundary. The tuned circuit often incorporates the circuit to either apply the transmitted signal to the track or recover the received signal from the other end of the section.
Consider a railway with two block sections as in the diagram. Section 1 has frequency A injected at the left-hand end and received at the right-hand end. Section 2 continues from the right hand end of section 1 where frequency B is injected and then received at the right-hand end of section 2.
There is often a gap between where frequency A is received and frequency B is injected. This is referred to as a 'tuned zone' and is a section of track where the amplitude of frequency A reduces in the direction of section 2 and the amplitude of frequency B reduces in the direction of section 1. The tuned zone can be of the order of 20 m long.
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 fewer 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 are more vulnerable to lightning strikes.
CSEE UM71
CSEE (now part of MERMEC) UM71 is another kind of jointless track circuit. It uses 1700 Hz and 2300 Hz on one track and 2000 Hz and 2600 Hz on the other. To reduce the chance of stray currents causing a wrong side failure the modulation frequency is calculated by dividing the base frequency by 128. Different rates of modulation can be detected by equipment on the trains and used for automatic train control, so long as the transmitter end (Tx) is at the front of the train.
The EBItrack (formerly TI21) and Westinghouse FS2500 jointless track circuits are similar to the UM71.
Data pickup unit
A jointless track circuit such as the CSEE can be divided with a data pickup unit (DPU), which is cheaper than splitting it into two track circuits. 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 use of DPUs is for timing circuits. Each track circuit frequency has its own DPU tuned to that frequency. DPUs can be located almost anywhere; they overcome the limitation that jointless tracks have a minimum length.
DC coded track circuits In non-electrified areas, DC coded track circuits may be used. These modulate the current going from the powersource end to the relay end and control the signals and cab signals without the need for line wires. The modulated currents can be detected by equipment connected to the track to provide signaling and indication information to activate proper cab signalling if available. They can be overlain by predictor systems to operate level crossings.
Brands of coded track circuit include: Alstom née GE Electrocode 5 Microtrax/E-Code by Hitachi née Union Switch & Signal
Cut tracks Where the length of a section exceeds the practical length of a track circuit, cut tracks can be provided. With a cut track, the relay of the last track cuts the powersource feed of the second last track circuit, and so on. Cut tracks are only suitable for unidirectional tracks.
Track circuits with ballast contamination will be shorter than those with good ballast, thus needing more cut tracks.
High-voltage tracks One common brand of high-voltage impulse track (HVIT) circuit is made by Jeumont-Schneider. The high voltage penetrates rust and other problems.
The HVIT transmits two pulses alternately, a narrow positive one at about 100 V DC and a negative broader one at about 30 V DC. The energy of the two pulses is the same. 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. The R-C circuits check that the positive and negative pulses are in correct phase. The two pulses operate at about 1 Hz.
The circuit operates on AC and DC electrified lines, with additional equipment.
Single rail and double rail
In non-electrified areas, insulated blockjoints come in pairs, one on each rail.
In electrified areas, a workaround is needed to allow the traction current, of the order of thousands of amperes, to return to the substation. This may be achieved by having no insulated block joints in one of the rails, called the return rail.
If both rails are needed to carry the heavy traction return current, then insulated blockjoints are provided in both rails, and impedance bonds are provided to carry the traction current around the insulated joints. Impedance bonds are essentially centre-tapped coils, which offer low impedance to traction current at say 50 Hz, while offering high impedance to signalling current at say 1.7 kHz.
Failure modes and prevention
Wheels and brakes Railway wheels are made from steel and provide a good short circuit from rail to rail (shunt resistance).
Longer trains with more wheels have better conductivity. Short trains, light railmotors or single engines can be a problem. In New Zealand there were problems with light Rm class railmotors not always being detected. Trains with a single Budd railmotor, which are also lightweight, and with discbrakes, had some problems when they stopped, and had to make a double stop to ensure good contact with the rails.
Cast iron brake shoes tend to clean the wheels of non-conductive debris (such as leaves and sand-based traction compounds), while disc brakes do not. As a result, some disc-braked vehicles have "scrubber pads" cleaning the wheels to aid in proper track circuit operation.
Relays Track circuit relays, referred to by signal maintainers as "vital relays," are specially designed to reduce the chance of wrong-side failures. They may, for example, have carbon-silver contacts to reduce the likelihood of the wrong contacts welding shut after power surges and lightning strikes.
Circuit failures The circuit is designed so the vast majority of failures will cause a "track occupied" indication (known as a right-side failure). Examples include: 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) 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. One perspex case warped in the heat, and touched the relay contacts, holding them up. Another Q-series "miniature" relay saw a metal washer slip off and jam the relay contacts up; the half-washers had to be replaced by full-circle washers.