The most common form of track circuit used is the detection of a train by the closing of an electrical circuit between the two rails because of the conducting nature of the rolling stock. This circuit may use DC in the simplest form, or may use AC, at various frequencies or with coded pulses.

DC Track Circuit (double-rail)

Double-rail DC track circuits are generally found only in non-electrified sections, and only where there is no concern with stray currents circulating in the earth or in the rails. The track circuit consists of a portion of the track which is insulated from the rest of the track by means of insulated rail joints. Within the section so insulated, bonding wires are provided to maintain good conductivity between adjacent rails. The rails on one side are insulated from those on the other by the use of wooden or other non-metallic sleepers. The track relay is connected across the two rails at one end of the track-circuited section, and a DC power source (track battery) is connected across the rails at the other end along with a regulating resistance. When there is no train in the section, the circuit is completed through the track relay which is therefore energized. The energization of the relay lights an appropriate indicator lamp in the cabin, but may also pull a signal off for entry to the section. When a train enters the section, it shunts the current through the track relay, which as a consequence is de-energized, leading in turn to an appropriate indication at the cabin, and to signals controlling entry to the section being set to danger and locked from being pulled off. Further, note that if the track battery fails, or the bonding connectors between rails break, the relay is de-energized and these failure conditions also result in signals being set to danger. Where traffic is (mostly) unidirectional on a line, the track relay is placed at the entrance end, so that the relay is de-energized as soon as the train enters the section, and operation of the relay is not compromised by leakage currents and other problems. Leakage currents between the rails always exist, and can reach high levels when it rains and at other times, which has to be taken into consideration when designing the circuit and its operating current values. The ballast makes a difference, with broken stone being the best and cinder being a poor candidate because it holds a lot of moisture. Ballast resistance should usually not be less than 6.5 ohms/km when wet. The resistance of the rails and the bonding wires should be less than 0.5 ohm/km.

DC track circuits of this kind are simple and cheap to install. When there are stray currents found in a region, these track circuits are often very problematic to use. Even if they are used, the presence of stray currents usually severely limits the length of the track which can be part of the track circuit.

Insulated Rail Joints

For a DC track circuit to reliably detect the location of a train within its specified section, the section must be electrically isolated from adjacent track (the exception being with jointless track circuiting methods — see below). Special kinds of rail joints are used, known as glued joints or insulated rail joints. Usually a special 940mm-long fishplate is used, with 6 holes for fish bolts. Special high tensile strength fish bolts are used and the entire fishplate and bolt assembly is glued on to the joint, including the ‘end post’ at the joint, using an epoxy impregnated fabric in multiple layers. A typical glued joint is 6.5m long and is welded to the adjoining rails. The glue and fabric ensure that the rail sections on either side of the joint are electrically separated.

Rail bonds

At normal joints within a track circuit section, electrical continuity must be ensured. Usually, one or two bonding wires are provided that connect the two rails across a fishplated joint. This is done even though the fishplate normally provides electrical continuity, to allow permanent way operations that involve unbolting the fishplates to continue without interfering with track circuiting. Also, dirt and surface impurities can cause the bolted fishplate joint not to conduct electricity reliably for track circuiting purposes (especially with AFTC or HFTC where the impedance of the joint to the particular frequencies involved is critical).

Single-rail DC Track Circuit

In 25kV AC electrified areas, single-rail DC track circuits can be used. Rails on one side of the track are used for the the returning traction current, with adjacent rails being bonded together for conductivity. Rails on the other side are bonded together in the section of the track circuit, but insulated at either end. The track relay and track battery are connected across the rails within the track circuit section as usual, but with high impedance chokes to prevent the traction current from flowing into the relay or battery. High-voltage fuses are also provided to protect the track circuit equipment from accidentally getting 25kV across it from a downed contact wire. The principle of operation is the same as with the plain two-rail DC track circuit described above. As a further safety measure, a high-voltage fuse or ‘interval of discharge’ (a device like a lightning protector) is provided across the rails so that if the contact wire breaks and falls on the rails, the insulated rail gets connected to the uninsulated one and is therefore earthed. The uninsulated rail on one track is bonded to the uninsulated rail of an adjacent track circuit as well, to provide a path for traction currents if the uninsulated rail breaks. Beyond the track circuit area, tranverse rail bonding connects the rails on either side together – this distributes the traction current better across both rails outside the track circuit.

This system is simple and cheap to install, but has some disadvantages. If one insulated rail joint fails, the track circuit is effectively expanded and will interfere with the operation of adjacent track circuits. Return traction currents or stray currents cause a longitudinal voltage drop across the uninsulated rail, which limits the length of the track circuited section. Finally, in contrast to AC track circuits that use impedance bonds and double insulated rails, a single broken rail cannot be easily detected by the imbalance of return currents.

Coded Track Circuit

A coded track circuit is a variation on the simple DC track circuit, where instead of a steady DC signal, a pulse-coded current is used for the track circuit. The pulse train is generated by a code transmitter. The track relay is energized and de-energized by the pulse train, and controls the current in a decoding transformer correspondingly by switching its taps. In turn, a track detector relay follows the changing current in the decoding transformer and is configured such that it is energized as long as the specific pulse pattern is being transmitted. When the train enters the zone of the track circuit and shunts the circuit, this relay is de-energized, and signals are brought to danger. Coded track circuits have a big advantage over the basic DC track circuits in that they are much less vulnerable to stray currents.

A further advantage of coded track circuits is that different codes can be used at different times to control the signal aspects. For instance, the signal governing entry to a section of track can be set to clear or caution depending on the pulse pattern used. With any of the pulse patterns being used, however, the presence of a train will still shunt the circuit and cause the signal to revert to danger. Track circuits can also be ‘overlaid’ or have some portions of track in common. In addition, the same coded pulse trains can also be picked up by locomotive equipment by induction to provide in-cab display of signal aspects.

AC Track Circuit

AC track circuits use an AC signal instead of a DC in the track circuit. The frequency used for the track circuit signal is usually 83.5Hz, to avoid interference from the 50Hz traction current. Their principal advantage is that they are immune to interference from stray currents, so that they can be quite long, up to 5km or so. AC track circuits may be used on unelectrified or electrified tracks. Impedance bonds see below are provided for the rails at the ends of the track circuit. The purpose of the impedance bonds is to provide a path of low impedance for traction currents to flow in both rails, and to provide high impedance and therefore block the AC signalling current. A band-pass filter and rectifier are used to extract a DC signal from the AC track circuit current, for the operation of the track relay. Apart from the fact that an AC signal is used, the general principle of operation is the same as with DC track circuits.

AC track circuits tend to be immune to stray currents and hence can be more reliable, and can be quite long – a few km in length. However, the provision of a separate 83.5Hz supply makes the installation costly.

Impedance Bonds

Impedance bonds used for AC track circuits consist of two low-resistance windings wound in opposite directions on a laminated iron core. Each winding is connected across the rails on either side of the track, and centre taps from each winding are connected together. With DC traction, under normal circumstances equal currents flow in each half of each winding and if the traction currents are evenly distributed across the two rails, there is no resultant flux in the iron core. In this state, when the core is not magnetized, it presents a path of high impedance to the track circuit current. In the case of an imbalance, the core would be magnetized to saturation and the track circuit current would no longer be faced with a high-impedance path; therefore, an air gap is introduced in the magnetic circuit to prevent saturation, and the impedance bond presents high impedance to the track circuit current in all cases up to about 20% traction current imbalance. With AC traction, when the traction currents are unbalanced, the half coil that carries more current induces an e.m.f. in the opposite half coil that tends to equalize the current. So air gaps are not generally necessary for AC traction. The impedance of the bond to the signalling current can be further increased by adding a secondary coil and a capacitor across it, in what is known as a resonated impedance bond. The secondary coil steps up the voltage and allows the use of a smaller capacitor than would otherwise be required. Auto-coupled impedance bonds are a modification of the resonated impedance bond idea. Here the winding across the rails in the track circuit zone forms one part of the winding of an auto-transformer, the other part having the capacitor in series. On one side of the track circuit, the other part of the auto-transformer is connected to the supply (100V) thereby being stepped down for the track circuit current, and the auto-transformer winding on the other side of the track circuit is connected to the track relay such that the track circuit current is stepped up to operate the relay. Thus, the current flowing in the bonds is usefully employed in operating the relay.

Audio Frequency Track Circuit

Most zones now have many sections that use AFTC, or Audio-Frequency Track Circuits, that are like the AC track circuits described above, but using a signalling frequency that is higher. Many frequencies are used. In early systems, 175Hz, 225Hz, 270Hz, 320Hz, and 831.33Hz were common. Multiples of 50Hz were avoided so that there is no interference from harmonics of the common line frequency for other electrical equipment or the AC traction supply. Today, there are many different systems. ABB equipment uses 1549Hz, 1699Hz, 1848Hz, 1996Hz, 2146Hz, 2296Hz, 2445Hz, and 2593Hz. Siemens equipment uses 4.75kHz, 5.25kHz, 5.75kHz, 6.25kHz, 9.5kHz, 10.5kHz, 11.5kHz, 12.5kHz, 13.5kHz, 14.5kHz, 15.5kHz, and 16.5kHz. Siemens equipment uses 1700Hz, 2000Hz, 2300Hz, and 2600Hz. A variant known as DC-coded AFTC from Alstom uses frequencies like 2100Hz, 2500Hz, 2900Hz, 3300Hz, 3700Hz, and 4100Hz.

AFTC is more reliable, especially where both DC and AC traction are in use, and allows the track circuit length to be increased a lot. The pioneers in adopting AFTC over simple DC or low-frequency AC track-circuiting were WR, SR, and CR (Dombivli, Pune-Lonavala, Chennai-Tambaram, Anand-Vatva, etc.). As with the low-frequency AC track circuits, a band-pass filter and a rectifier are used to extract the signal; however, in many cases an amplifier is needed to strengthen the signal.

High Frequency Track Circuit

As the name implies, High Frequency Track Circuits (HFTC) use substantially higher frequencies, e.g., 40kHz, for the track circuit current. This kind of track circuit operates a little differently from the other AC track circuit types. Impedance bonds are not used. Instead, at either end of the track circuit, rail-to-rail shorts are provided. A signal transmitter that generates the high frequency signal is connected to the rails at one end using an adapting transformer, which has one winding across the rails with a capacitor in series, while the transmitter is connected across the other winding. Similarly, a receiver is connected across the rails at the other end using another adapting transformer. The transmitter and receiver connections are a little distance (5m or so) inside from the rail-to-rail shorts. The receiver usually includes a tuned filter, rectifier, and amplifier for the signal frequency. Electrically, the track circuit zone inside the rail-to-rail shorts looks like two tuned LC circuits in parallel, with the inductance of the enclosed section of track in between them in series. The capacitors are adjusted so that the enclosed section of track is tuned to the track circuit frequency. When no train is on the track, the signal from the transmitter is received and detected at the receiver, and is used (via generation of a DC control voltage) to keep the track relay energized. When a train approaches the track circuit, it shunts the track circuit and – depending on the positions of the wheels – either de-tunes the circuit or shorts the transmitter or receiver (or both). Any of these cause the track relay to be de-energized.

In a variation on the above, the transmitter may generate pulse trains of specified duration and patterns with the high frequency signal. These are detected and converted to square waves which activate a peak detector, which in turn controls the generation of the DC control voltage to energize the track relay. In this scheme, different coded pulse trains can be used to control different signalling aspects.

The rail-to-rail shorts define the limits of the track circuit and therefore the circuit is immune to interference from adjacent track circuits. Also, the LC circuit on the receiver side can be tuned very specifically to the track circuit frequency, so that other signalling applications that use other frequencies can be used on the same section of track without compromising the track circuit’s operation.

CR was the first zone on IR to experiment with HFTC.

Jeumont Track Circuit

The Jeumont Track Circuit (or Jeumont-Schneider track circuit) is a design that has been tried in areas where environmental conditions make it hard to get good contact between rails and wheels, reducing the shunting effect of a train on the track. Often, in such conditions, a higher track circuit voltage helps as the track circuit current can break down and go through thin films of oxides, salts, coal dust, scale, etc., on the surfaces of the rails. However, constant or steady AC high voltages lead to higher leakage currents and therefore waste power. In the Jeumont design asymmetric high voltage pulses with a small duty cycle are used – with a high peak on the positive side (3ms), and a low amplitude negative cycle (17ms). The pulses are emitted at about 3Hz frequency. The low duty cycle keeps power consumption low. A specialized detector rejects symmetric signals (as from the traction currents) and detects the asymmetric pulses. These track circuits were in use at the Tambaram yards of SR, and in some areas around Kolkata. They are especially suitable in areas with DC traction (but can be used where both DC and AC traction are used) because of the corrosion problems in DC traction areas. They are also suitable for use in tunnels and other areas where oxidation of rails is more intense. As with DC track circuits, slight variants are possible for single-rail or double-rail returns in electrified or unelectrified sections. Some other variations of the basic principle exist – IR literature also refers to some designs as Pulsed High Voltage Track Circuits.

Jointless Track Circuits

The Aster Track Circuit (also referred to as Jointless Audio Frequency Track Circuit) is a design suitable for use in areas with long welded rail where impedance bonds or insulated joints cannot be provided. It uses signalling frequencies around 2.2kHz or so. An ‘electric boundary joint’ is created by connecting two capacitors in series across the rails at either end of the track circuit zone. A ‘rejector’ cable is connected from one rail about 11m outside the track circuit to the point between the two capacitors, and then to the other rail about 18m inside the track circuit. The capacitors in conjunction with the inductances of the rails and the rejector cable form tuned circuits. In addition, the transmitter for the track circuit frequency is connected across one of the capacitors. This leads to the impressed voltage being available in full inside the track circuit zone, but reduced (usually by half) outside the track circuit. The receiver for the track circuit is connected across the capacitor at the other end of the track circuit, on the side connected to the other rail. Adjacent track circuits use the same principle (the next track circuit has its receiver across the other capacitor at the end where the transmitter of the first circuit is connected.)

In the diagram showing the Aster track circuit, note that the lengths of rail across which the cross-connection T-W-Q-X-S are made are 11m (T-P) and 18m (R-S) respectively. This cross-connection forms the ‘rejector circuit’ or ‘electric boundary joint’. The equivalent circuit is shown at bottom right in the diagram. Note that the track circuit signal is injected across Q-R, and faces a circuit with Ct in parallel with inductances QX and RS. This circuit can therefore be set up so it offers high impedance to the signal frequency, preventing it from propagating it to the track circuit section on the right. Meanwhile, the circuit Q-W-T-P can be set up allow the signal frequency to propagate to the left. Adjacent track circuits can use different frequencies and reduce interference by means of these rejector circuit arrangements. In addition, note that the voltage Va impressed across QR is available across TU to the left, but gets dropped (to Va/2) across the tracks on the right. The same applies to the voltage Vb applied by the track circuit transmitter for ‘B’ to the left. See the graph plotting the net voltage as we move from one end of the track circuit (‘A’, the region in the middle) to another – the voltage from track circuit ‘B’ (to the left) is reduced in half, Vb/2, on the left end of track circuit ‘A’, and falls essentially to zero at the right end of ‘A’. The voltage from track circuit ‘A’ starts at about zero on the left end, and rises to its full value Va on the right. The net voltage therefore rises as we go from the left to the right from Vb/2 to Va.

Other kinds of jointless track circuits exist, where the detection of the section occupancy by a train is done by measuring the attenuation of the signal which is at a frequency (about 10kHz, usually) which undergoes significant attenuation in rails over the distances of interest and whose propagation characteristics are known. This also means that the entrance of a train into the track circuit section is not determined precisely based on its position — instead, safety factors are incorporated in the calculations to yield zones within which train occupancy can be determined in a guaranteed fashion. The system can be made even more reliable by coding the signal in a pulse train allowing the receiver to distinguish between signals of different track circuit sections (see below).

In a variation of the jointless track-circuiting scheme, trackside units can be used to set up a resonant circuit and constrain the signal (usually between 1.5kHz and 3kHz) to a particular section of track. The advantage of jointless AFTC is clear in that insulated joints are not required, reducing maintenance, allowing the use of long welded rail sections, and eliminating the problems of insulated joint failures. Jointless AFTC sections can be 1-1.5km in length.

Coded Jointless AFTC

This system uses a mechanism such as the Aster design (see above) where insulated rail joints or impedance bonds are not needed. In addition, rather than using single frequencies for the track circuit current, coded pulse trains are used, which further reduces interference between adjacent track circuits. Jointless AFTC is in use on the Delhi Metro, and a few other places on the IR network. It was first introduced on the Tambaram – Madras Beach section of SR. AFTC equipment used by IR is from Adtranz, Siemens, US&S, and Alsthom. Jointless AFTC units are manufactured in India by Medha Ltd.

Source – IFRCA.org

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