Track-circuiting is mandatory in sections where visibility is a problem, shunting operations are routinely carried out on the block section outside station limits on the main running line, or if special situations exist, e.g., if the advanced starter is more than one full train-length ahead of the most advanced trailing points of the station.
The track circuit provides additional functionality for detecting broken rails, though only to a limited extent in AC traction areas and not in the common rail in DC traction areas. Axle counters, however, offer no such facility. However, experience has shown that broken rails often occur near the insulated block joints which are used to electrically isolate adjacent track circuits. Since axle counters do not require such block joints, the risk of having broken rails is significantly reduced.
ELECTRICAL TRACK CIRCUIT :
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.
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 signaling 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.
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 energized 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-energized. Circuits through the relay contacts, therefore, report whether or not the track is occupied. In almost all railway electrification schemes the rails are used to carry the return current. This prevents the 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 signaling system.
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.For a track circuit to reliably detect the location of a train within its specified section, the section must be electrically isolated from the adjacent track (the exception being with joint less AFTC -see below). For this, IR uses special kinds of rail joints, known as glued joints, especially on LWR (long welded rail) sections.
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 are 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.
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 fishplate 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). In a few cases, special-purpose resonant bonds or other devices are provided at joints to allow particular track circuit signals (AFTC or HFTC) to flow while blocking others.
On most sections with track circuiting, track integrity checks are also provided with these circuits, which electrically detect a break in the circuit which would indicate a break or deformity in the rails. When interlocked with signals, this also prevents a signal from being pulled off if the track has a defect. Of course, not all kinds of track defects can be detected in this manner.