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

Recapitulation

Recapitulation.

The rolling stock protects itself by putting the signal in its immediate rear to danger on double lines, and on single lines by also putting the opposing signals to danger as far as the next crossing place. This happy result is attained because the train short-circuits the track, which is a current-carrying circuit, and cuts off current (juice in the vernacular) to the relay, which is operated by the track; and the relay (in its de-energised condition) switches “in” the red light to the signal passed. Very simple! but wait! On single lines, all the opposing signals ahead to the next crossing place must also go to danger before the train receives its departure signal. Well! Before the departure signal is cleared the current to the relays controlling the opposing signals is cut off, causing them to go to “danger.” Each section of track between signals, as the train reaches it, also cuts off current to the opposing signals, which remain at “danger” until passed.

The overhead control wires on single lines provide for this and other necessary movements. On both single and double lines the second page 37 signals behind the trains successively return to clear, so that a following train can advance at the proper interval. Of course it is not quite so simple as that; but the tangle of wires and instruments seems to know that this is the result expected and, properly fed and tuned up, they certainly manage!

In actual practice the simple system detailed above is complicated by the special work required at branches, stations, wayside sidings, unattended crossing loops; and for level crossing warning devices, linking up with tablet systems, lock and block, mechanically equipped stations, etc. All these require special treatment according to the facilities desired, and come under the general term of “interlocking,” which will be the subject for the next article.