If a yard or siding area often shunts past a signal and back into those sidings, a provision may be made for opposing locking omitted. This is where two shunt signals read towards each other and may both have a route set at the same time. The signals remain at proceed until cancelled, even with trains passing between the signals. Once one of the routes is cancelled (and times out), the signals revert back to normal operation, cancelling themselves if a train passes if required by the interlocking.
Last edited by GeoffM on 15/09/2016 at 03:01