Track Circuits Overview and Requirements

The electrical resistance of a track circuit, up to 1.5 ohms per 1000 meters for tracks less than 700 meters.

Connection of rail sections to ensure uniform electrical potential for reliable operation of track circuits.

Point where a track circuit ends, set before the 'Fouling Mark' to prevent vehicle interference.

Point where the minimum distance of 3 meters from the track circuit termination must exist, to prevent vehicles interfering with the track circuit.

Track circuits used in separated track sections, where the track relays' status controls the feed and shunt.

A break in the electrical continuity of the track, used to isolate sections.

Avoid placing insulated rail joints on outer rail curves to minimize dead zones (areas where standing vehicles aren't detected).

QBAT relays, with a biased magnetic arrangement, are more resistant to alternating current interference. This allows them to be utilized in track circuits up to 750 meters with an additional 'B' type choke.

DC track circuits can be maintained with higher catenary currents (up to 800 amps single-track, 1000 amps double-track) by adding a 'B' type choke. This is a temporary solution.

Before implementing single rail DC track circuits in AC electrified areas, measure stray DC currents. Limits exist based on track circuit length.

Audio Frequency track circuits in areas with railway electrification (RE) use impedance bonds at IRJs (Insulating Rail Joints) and separate earth wires are needed for overhead line structures to reduce interference.

A choke coil, of an approved type, must be used in series with the feed resistance to prevent rail voltage from affecting the track circuit equipment.

In double-rail track circuits, impedance bonds are necessary at the rail joints to address return current, as both rails carry traction return current.

Audio Frequency Track Circuit (AFTC) must be configured as a double rail track circuit, balancing both rails to earth and current flow.

Impedance bonds are used in AFTC when the design doesn't allow for proper rail balancing of traction return current.

AC impedance bonds can handle 400 Amperes continuously (200 Amps per rail) and 1000 Amperes peak for 5 minutes without damage.

DC impedance bonds have higher current ratings than AC bonds, but are larger and more costly. AC bonds are suitable only for AC systems.

Leads connecting impedance bonds must be PVC-sheathed cables with compression lugs securely bolted to the bonds, capable of carrying the traction current.

The S&T department is responsible for maintaining connections between rails and impedance bonds, along with connections between different bonds on the same track.

Installation and design of impedance bonds must accommodate track maintenance and avoid safety hazards for personnel working near the track.

Adjacent axle detectors should be at least 2 meters apart to prevent interference.

Protective shields on axle detectors prevent damage from train parts.

Track-side electronics should be close to detectors to reduce cable length.

A board to terminate incoming cables used for testing.

In some cases, transmitter and receiver cables should be placed in separate pipes.

Track devices should have a minimum 400 mm sleeper spacing to avoid vibrations.

Central power supply (DC-DC converter) and battery backup are required for stable operation.

Install the evaluator in a secure, non-public location, above flood level, away from interference.

The process of resetting an axle counter to a known state after operation.

A situation where an axle counter cannot be reset or fails repeatedly after resetting.

Resetting the systems of the Block Section after matching 'count in' and 'count out'.

The staff member responsible for monitoring and resetting aspects of the system during operation.

Routine upkeep of axle counters following RDSO guidelines and manufacturer recommendations.

Uneven movement of rails, potentially affecting axle counter inter-distance accuracy.

Necessary anchoring of track to prevent differential creep.

Tie tampers cannot be used for maintenance around axle counter installations affecting 4 sleepers on either side.

Correct replacement of axle counter components following manufacturer's technical manuals.

Avoidance of maintenance requiring track device disconnection and counter parameter adjustments in the operational system.

Before indoor/outdoor maintenance on Axle Counters, maintenance staff must disconnect and de-energize related sections.

After maintenance, re-energize the sections, checking parameters and physical clearance before reconnecting.

Activities like parameter measurement or cleaning, that don't affect system operation, can proceed without disconnection.

Routine checks to confirm proper fitting, tightness, and connections of track devices and axle detectors.

If any damage or issues are found on trackside equipment, the associated track section needs to be disconnected before addressing the issue.

Where possible, using dataloggers with interfaces allows for monitoring the operation of axle counters.

Appendix I contains schedules for different rail track components testing and maintenance.

Annexure 17-A1 details the measurement of stray DC current before installing DC single rail track circuits.

Supplementary documents referenced for complete details

Appendix I contains maintenance schedules covering various axle counter types.

For berthing tracks with multiple sections, individual track circuit indications should be shown on the panel.

At stations other than berthing tracks, the indicator should be yellow/white when the line is unoccupied, and red when occupied.

The resistance of DC track circuits must be adjustable and of an approved type.

In future installations, only plug-in type track relays (9 ohms) should be used in both Non-RE and RE areas.

Q series track relays should be used with Q series slow-to-pick-up relays as repeater relays.

DC track relays should be excited at a minimum of 125% of their rated pick-up voltage, considering minimum ballast resistance and normal supply voltage. Maximum excitation should not exceed 235% for QBAT relays and 300% for other plug-in relays.

The maximum length of a track circuit depends on track parameters (min. max sleeper type/resistance, etc.) Refer to the table for limits.

TUs and ETUs should be mounted at least two meters away from the nearest rail to ensure staff safety.

If a track circuit fails, investigate the cause of receiver voltage drift before adjusting the receiver's gain.

Tune the impedance bond within the track circuit by matching the resonating capacitor's setting to the track circuit frequency.

Track circuit length depends on ballast resistance, frequency, layout (e.g., level crossings), and AFTC vendor; it's broadly limited to 700 meters in end-fed mode.

Audio Frequency Track Circuits (AFTC) shall not be used for new construction; existing AFTCs should be maintained.

AC electrified track circuits can use IRJs or ESJs and be set up as single- or double-rail systems. Double rail is needed for ESJ systems.

For single-rail DC track circuits, one rail carries the return current. Connections to overhead lines, return feeding points, and boosters should only be made to this rail.

The rail adjacent to the overhead line mast is usually preferred as the uninsulated return rail, but this isn't always practical.

Connecting track circuit rails in series to prevent malfunctions from affecting the system's operation.

Traction return rail bonding is not possible due to traction considerations.

Connecting adjacent uninsulated rails to limit current flow in case of a rail break.

Provides a continuous earth connection for traction return current on single-line track circuits.

Structure bonds connecting uninsulated rails to traction masts must be riveted at both ends.

No structure bonds or other connections are allowed to running rails in double rail track circuits.

Devices that count train axles passing over a section of track.

A type of signaling system where trains are stopped at predetermined points.

A signaling system that automatically controls train movements through blocks.

An axle counter using digital technology.

A system to prevent axle counters from counting trolleys.

Procedures for properly installing axle counters as specified by the manufacturer.

A track circuit where the rails form a complete electrical loop.

The minimum distance between two adjacent axle detectors of different axle counters to minimize interference, typically at least 2 meters or as per OEM manuals.

Protective shields on axle detectors to prevent damage from hanging parts of moving trains.

Track-side electronic equipment should be located close to axle detectors to minimize cable length.

A board used to terminate incoming cables from axle detectors and evaluators for testing purposes.

In some cases, the transmitter and receiver coil cables between the axle detector and associated track-side electronics should be routed in different pipes.

Track devices (sensors) should have a minimum spacing of 400 mm between sleepers.

Track-side electronic equipment should be powered from a central power supply, like a DC-DC converter, with adequate battery backup.

The evaluator should be installed in a secure location, preferably a relay room, inaccessible to unauthorized personnel, above flood level, and protected from rain.

The process of returning an axle counter to a known operating state after a failure.

A situation where an axle counter malfunctions, does not reset properly, or repeatedly shows a fault condition.

Resetting the axle counter requires collaboration between dispatching and receiving station staff.

Verifying block section clearance for resetting the axle counter, done by exchanging private identification numbers between dispatching and receiving stations.

A logbook where activities like axle counter resets and verification are documented by both stations.

The staff member in charge of train dispatching or receiving and who is responsible for the procedures related to axle counter resetting.

Checking that a section of track is clear of trains or obstructions before an axle counter operation is performed.

A device used to verify that a specific section of track is physically clear for train operational purposes.

The key to operate the axle counter reset mechanism is kept locked away in a secure location.

A counter that records each activation of the reset button, but cannot be manually reset or manipulated.

Official guidelines specifying the procedures for resetting an axle counter and arranging train movements, keeping safety in mind.