Track circuits on the third rail network

On DC electrified lines, both running rails must be used to return the high traction current from an electric train to the sub-station which generated it. The rails must often be shared with track circuits. The track circuits are AC, so will not be falsely energised by the returning DC traction current. Abutting track circuits are traditionally separated by insulated joints, and these would obstruct the flow of the DC traction current, so a bypass path has to be engineered at each interface between adjacent track circuits.

track circuit
Two plain line track circuits, showing the equipment cabinets and cross bonding

Photograph by Paul Hepworth

This is done with impedance bonds, as shown in the photo above. These act as a short circuit to the DC traction current, but use resonance to act as an electrical "brick wall" to AC supplies. The DC can flow from both rails, via the impedance bond (sunk into the four foot) into the sleeper mounted metal busbar plate. It bypasses the insulated rail joints, and next enters the impedance bond of the adjacent track circuit, then flows out to both rails. The track circuit's AC supplies cannot follow, and are directed instead into cables which link to trackside equipment cupboards, where the track circuit feed and relays are housed. The above photo also features a similar installation on the adjacent line to the left of the shot. The two metal busbar plates are bonded together. This "cross bonding" allows the traction current to be shared between four rails, rather than two, so reduces voltage drop on the third rail.

track circuit
Track circuit insulation arrangement for pointwork

Taken from a photograph by Steve Jones at with permission stated to be granted for non-commercial use. This image will be removed if deemed necessary by the copyright holder.

Through points, one rail is insulated for track circuit separation, and the returning traction current is confined to one rail for a short distance. The picture above, taken at Wandsworth Road, shows an abutment between a plain line track circuit and one through points (out of shot to the left). The latter track circuit does not have an impedance bond, and the busbar plate is bonded to one rail only. Where possible, the third rail is placed alongside this rail, so that an emergency short circuiting bar can be applied to cut off the traction supply.

All photographs are copyright

This page was last updated 18 July 2005, and created from a text by Paul Hepworth

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