Far-End Shunt Failure on a Coded Track Circuit
At 14:05 on a dry July afternoon you are testing the 4,800-ft coded track circuit in Block 118-2, MP 118.4 to 119.6, on a single-track ABS subdivision where freight is limited to 49 mph. With the block unoccupied the track relay is picked at 4.2 mA and the feed resistor is tapped at 0.9 ohm at the west end; the last test sheet, 11 months ago, recorded a 2.0-ohm tap. Your 0.06-ohm test shunt placed 100 ft from the west feed does not drop the relay. A shunt 100 ft from the east relay end does drop it. The weather has been dry for two weeks and ballast resistance measures 55 ohms per 1,000 ft. The block must be given back for an eastbound coal train in 40 minutes. Your foreman, who has never seen a feed-end failure, says, "Tap the feed down one notch and sign the test sheet; the train can't wait."
1. Explain what the failed feed-end shunt test tells you about this circuit, and evaluate the foreman's instruction.
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The 0.06-ohm shunt is the standard stand-in for the worst case you must detect: a single car, or rusty rail under a train. A shunt at the relay end works because it short-circuits the relay directly, so current through the relay collapses. A shunt at the feed end has to divert current through the rails and the ballast path, so whether the relay drops depends on the margin between the feed current reaching the relay and the relay's drop-away current.
Dry ballast (55 ohms per 1,000 ft is high) means very little leakage, so almost the full feed current arrives at the relay and the drop-away margin is thin. The circuit was re-tapped from 2.0 ohm to 0.9 ohm, which raised the feed current. Tapping it down further, as the foreman wants, lowers feed resistance and pushes even more current through the shunt and into the relay. That moves the failure in exactly the wrong direction.
The correct direction is to reduce energy at the feed - raise the tap or reduce feed voltage - until the feed-end shunt drops the relay with margin while the relay still picks reliably under the wettest ballast condition. Then re-verify both ends and mid-block. Signing the existing sheet with no passing feed-end test would be a false record and hides a false-clear hazard.
2. What do you do with the block and the eastbound coal train before any repair? Put the actions in order and explain why a speed restriction alone is not the answer.
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Order matters: protect first, then notify, then repair.
- Do not sign the test sheet and do not report the block as tested.
- Tell the dispatcher immediately that the block's shunting sensitivity fails at the feed end, so the track circuit cannot be trusted to show occupancy at that end.
- Have the block removed from service as a track-circuit-controlled block - manual/absolute block, or signal suspension with the railroad's mandatory directive, whichever the current rules require. No following movement may be authorized on the strength of that circuit.
- Notify your supervisor and open the record entry; a track circuit that can fail to detect an occupancy is a false-proceed condition and is normally reportable.
A speed restriction on the coal train does not fix the hazard. The danger is not the coal train's speed; it is a following train receiving a clear signal into a block where the standing train is not being detected because its axles are not shunting reliably at the feed end. Protecting against that requires removing reliance on the circuit, not slowing one train. Restricted speed on the following movement can be part of the interim protection only if the rule is applied to every movement that would otherwise use the signal as authority.
3. The feed is re-tapped and the circuit now passes. What test sequence and records close this out, and what would you look for as the root cause?
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Sequence after adjustment:
- Confirm the relay pick-up with no shunt at the highest expected ballast resistance and confirm it drops under the lowest expected ballast resistance (wet weather) so you are not trading a false clear for chronic false occupancy.
- Apply the 0.06-ohm shunt at the feed end, at the relay end, at mid-block, and at roughly 100 ft from each end. All points must drop the relay.
- Check the relay drop-away current against pick-up and confirm the relay is the correct part number for the circuit.
- Inspect bonds and connections at the feed end for a broken or high-resistance bond, since that also shifts current distribution; look for signs someone re-tapped the feed.
Records: record the new tap, measured relay currents, ballast reading, weather, shunt locations, date, and your name on the test form, and note the failure and the corrective action in the signal maintenance record.
Root cause: the tap change from 2.0 to 0.9 ohm is the leading explanation. Finding out who changed it and when is part of the close-out, because the same unauthorized adjustment may exist elsewhere on the territory.