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Signalling and Interlocking.

Probably no branch of engineering has developed more rapidly and been subject to more changes than signalling during the past 15 years. The development from the older type of mechanically operated signals and points, with which most people are familiar, to the latest development of centralised traffic control, by means of which points and signals are operated from a central point over lengths of line of 50 miles and more, is a very great advance. These developments have been possible only through the perfecting of signal apparatus and the adoption of many of the most ingenious inventions. Modern signal apparatus is complicated, but it can, when properly installed, do almost anything except think.

As is generally conceded, signals provide for the safety of traffic, but in addition to this, signals assist in the expeditious control of traffic. This latter function is not always quite so clearly appreciated.

The New Zealand Railways have followed the development of signalling fairly closely and, although at the present time there are no installations of centralised traffic control, automatic signalling has been installed over 318 miles of the railway. In addition to this, there are 107 mechanical interlockings for operating yards, and 27 interlockings operated by electric or pneumatic power. All facing points on the Main Line between Auckland and Invercargill not controlled by complete interlocking are key locked, and detected in such a way that the safety of the Main Line is assured.

To most people signalling only means a series of posts by the side of the railway, at any one of which a train stops when the signal is at “danger.” There is little realization of the complicated control and apparatus—usually out of sight—by means of which the indications of the signals are controlled to ensure reliable working, to continually safeguard the passage of trains, and in all weathers and at all times to give the necessary indications and assurance to the engine-driver that he can safely maintain the speed of his train, knowing that his road is safe.

Supplementing the signals, the tablet system, which has been installed throughout the Main Lines and on the major Branches, gives assurance to the driver that his section is clear for the passage of his train.

A description of the possibilities of signalling in expediting traffic would require too much space to be dealt with here, but there is one phase of the question which may be touched upon, and that is, that it is only by close co-operation and by making the fullest use of the facilities afforded by signalling that the best results can be obtained in traffic working.

In order that the best and most flexible schemes may be devised, it is necessary that the very fullest information should always be available in regard to the traffic operations which the signalling is required to control.

It is sometimes stated that signalling costs a good deal of money, and this is true to the extent that reliable apparatus cannot be cheap; but the decision as to whether anything is expensive or cheap must be measured by results.

The results of signalling are safety and efficiency in traffic operation — both almost impossible to calculate in pounds, shillings and pence, but nevertheless real; and railways throughout the world, even in these financially difficult times, are still installing signalling, particularly of the automatic type, and to quote a leading English authority upon economic working, “Signals mean Salvation.”