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The New Zealand Railways Magazine, Volume 2, Issue 7 (November 1, 1927)

Signalling. — Interlocking

page 30


The Locking Plate.

The designing of the locking plate is one of the few relaxations permitted to the signal expert.

The correctly designed locking plate provides that the signal and points levers have their movements so controlled and their grouping so selected that no signal lever can be pulled to “clear” unless the correct route is set, no shunt can be signalled unless provided for, no two conflicting movements can be signalled together, no facing point can be left unlocked on the ground for a train route, and no trap point left open on the road signalled.

Electric Tablet Machines

Electric Tablet Machines

Some half dozen different types of locking are in use in New Zealand, but the form known as “Tappet locking” has gradually ousted other competitiors, both for mechanical and electric machines, having proved in practice to possess those qualities of flexibility, strength, durability, and simplicity, that are necessary for a first grade safety appliance in railway working.

A Tappet locking plate appears on a mechanical machine as a wide shelf supported by brackets bolted to the frame of the machine and extending the whole length. The shelf consists of parallel troughs like a series of channel irons with their open sides upwards. In these troughs steel bars called “bridles” lie side by side. Rivetted to the bridles are triangular pieces of steel called “dogs,” the dogs on one bridle forming a combination which, when that bridle is moved along the trough, locks up a series of levers. The levers are brought into relation with the bridles by flat lengths of iron (“Sword irons”) pivoted on each lever, and as the lever is pulled, these slide at right angles across the bridles through grooves cut in the sides of all the troughs. These sword irons, therefore, lie across the path of the dogs rivetted to the bridles and have triangular notches cut in their sides into which the dogs fit when a bridle is moved (and the sword irons are in the correct position).

Each bridle is operated by one of the sword irons. In the normal position of the lever (and its attached sword iron), the driving dog stands in its notch. Upon the sword iron being moved, that dog is forced from its notch and so its bridle (armed with a series of dogs) engages the sword irons of conflicting levers and locks them until the operating lever is restored to normal.


The principal considerations are location and visibility.

Location means correct interval to allow of accommodation and train control; a situation where the best view is obtainable both by driver and signalman; a correct placing behind fouling points of conflicting routes; and protection for watering engines, etc.

Visibility means correct aspect in the case of bracket or gantry signals in relation to both height and spacing, to indicate divergent routes. The height of signals and the placing of shunting signals on posts where advisable also comes under this heading.

Facing Point Locks.

On all train running routes, at interlocked stations facing points are provided with a lock consisting of a plunger which locks the switches in either position as required before the signal governing that route can be set to clear.

A bar lying parallel to and alongside the rail is part of this lock, and this bar is required to rise above rail level in the act of locking or unlocking the points. It is long enough to cover any vehicle wheel base and so the points cannot be unlocked while any vehicle is passing.

page 31

Trap Points.

Generally a single rail trap point is used to protect running roads from vehicles left standing in sidings being blown or wrongly pushed from the sidings. As the normal position of such traps is open for the derail, such vehicles become derailed at the trap.

Shunting Signals.

A shunting signal consists of a round target either on the ground or elevated on a post. One shunting signal may indicate several routes, and appropriate locking is provided for each route. The intricacy of the locking plate is mainly due to the necessity for providing the maximum freedom of shunting movements consistent with safety, and alternative combinations of locking must be provided which become effective according to the routes set and the shunting signals used together. Such special locking combinations are collectively known as “permissive” locking.

Shewing distant control of motor-worked points. The signal cabin is seen beyond the water-tower.

Shewing distant control of motor-worked points. The signal cabin is seen beyond the water-tower.

Electric Interlocking.

As previously stated the essential difference in principle from mechanical interlocking consists of the interlocked area being track circuited, enabling automatic protection of running roads to be effective for train running movements. Shunting, however, can be given only a certain measure of automatic protection, on account of the necessity for the greatest flexibility in this work and the proved value of giving the trained signalman a certain amount of freedom.

As in Mechanical interlocking, the locking plate is the chief agent in preventing conflicting movements of signals, points, etc.

There is no mechanical connection between the interlocking machine and the points, traps, signals, etc. The operation of these units is affected through an electric circuit controlled by the lever. The points are moved by a motor, embodying a facing point lock and detecting devices, the motor being placed at the points.

Current cannot be supplied to operate the motor if a vehicle is standing on the points fouling the track and also any wrong adjustment or breakage of the permanent way will prevent the motor from operating until the defect is remedied.

Running signals, whether of “colour light” or motor worked arm pattern, are operated by electric circuits which are “switched in” by the levers in the cabin.

A signal operating circuit, in addition to passing through contacts on its own controlling lever, passes through contacts on all points levers used in setting up the correct route and also through the relay contacts of all tracks which will be fouled on that route.

All these contacts must be in the correct position or the signal will not clear.

Shunting signals are controlled in the same way, but their operating circuits do not always pass through the track relay contacts, as it is obvious that, in shunting, an engine may require to pass to an occupied road. The power required to operate the various units and instruments used is supplied from a transformer centrally situated or several transformers are provided at convenient points if the yard is over 24 chains in length. Main feed wires tapping these transformers are led through the yard, and current is drawn from these at convenient points (known as relay box locations) and at the signal cabin.

All the roads in the interlocked area are electrified and divided into separate tracks, so that each pair of points is situated in its own fouling track and, where points fouling tracks do not adjoin, the road between them is a separate track. All these tracks are shown on the illuminated diagram in the cabin, each with its own light which goes out when that track is occupied or fouled. The signalman can thus see at a glance the condition of his yard and can note whether his approach or departure tracks outside the station are occupied. The wires required for all this control are laid in wooden trunking throughout the yard and connected up to the various units at the cabin or in the relay boxes placed at intervals through the yard.