Locking Relays. Associated with each lever on the panel are a Normal locking relay and a Reverse locking relay. Only one of these relays can be energised at a time. It is possible for both to be de-energised.

Latching. Points lock relays latch either normal or reverse so that if power goes off the points do not change when it comes on again. The signal locking relay latches normal but not reverse so that the signalman can always put the signal to stop.

Signal Normal Lock Relay. This relay must be up for associated interlocking to be free. Signal Approach and Back locking is implemented by preventing the Normal Lock Relay from picking. 44 and 38 points both require 74 normal.

Signal Reverse Lock Relay. This relay must be up for a signal to show a proceed indication. The actual indication shown will depend on track relays and the position of points.

Type of Relay. Locking relays can be of various types, including magnetically latching relays or a two-coil relay with mechanical interlocking and latching.

Freeing the interlocking. The normal lock relay 74NLR must be up for the interlocking to be released. Approach and back locking works by preventing 74NLR from picking.

Selecting Locking Circuit. When 74 signal is cleared the lock stick (74LS) will be up, which selects the approach locking circuits. When a train passes the signal the lock stick (74LS) drops and selects the back locking circuit.

JR. If lock stick is up (train has not passed signal), approach circuit 77T is occupied and the lever put back to normal (74RLR down) 60 second timer JR will operate. When JR has run down it bypasses the approach locking circuits.

Modifying Locking. Approach locking is modified if 48 points are reverse or either 78 or 77 signals is cleared. Backlocking is modified if 44 points are reverse.

Lock Stick. If the signal is normal (74NLR up) and the track in advance is not occupied (44TPR up) the lock stick will operate and stick on its own front contact (74LS).

If the down main is switched out (2LRPR) the lock stick will pick whenever 44T is clear, regardless of the position of 74NLR.

When a train passes the signal 44TPR drops and the lock stick drops.

Reverse Lock Relay. 74RLR requires that either the lever on the panel be Reverse or that the down main be switched out (2RLR). In addition 74RLR will only pick if points are in the correct position.

Signal Lighting. If the signal is reverse (7RLR up), the lock stick (74LS) is up and the timer (74JR) is not operating the signal can show a proceed indication. The actual indication shown is determined by the position of 44 points and the state of the track circuits.

Flasher. If 74 signal is at stop (74RGPR), 74LS is down and the lever is not Normal the red light will be operated by 1DFR which is a flasher. This indicates that the lever is out of correspondance and should be put back to normal. If the lock stick (74LS) is up the flasher will be bypassed and the red light will not flash when the lever is reversed.