The electrical short circuit created when a solid metal wheelset bridges the two rails of a track circuit. This connection drops the track relay, indicating to the signal system and dispatcher that a train occupies the block. If a train fails to shunt the track due to rust or sand, the system may falsely show the block as clear.
Heard on the job
That rusty spur track won't hold a shunt, so don't trust the signals if you leave a cut of cars out there.
Insider tip
A loss of shunt is a major safety hazard, often caused by heavy sanding, leaf oil, or lightweight single cars on rusty rails.