scope:

Introduction

It is required to set two directly opposing shunt routes to allow a train to shunt backwards and forwards without further Signalman action once both signals have cleared.

Principles

The first of the two routes is set in the normal manner. The second route can then be set, with any opposing locking ignored, so long as a train has not passed the first signal. This ensures that the signal sticks and train in section proving are in the correct initial state.

Once both routes are set, both signals prove the track circuits in route clear at time of clearing only, and signal stick unsetting and train in section proving is inhibited. This ensures that the signal can not be replaced by track circuit occupation, and that, if button replaced, the thirty second time out of approach locking is enforced. Once one route is cancelled, the other signal reverts to normal operation.