[Illustration: Section of Saxby & Farmer Interlocking Machine.
(Showing two levers and locking mechanism. _A_ is normal, _B_ is reversed.)]
The arrangement of a group of signals and switches in such a way that their movements are made mutually dependent one upon the other, and so that it is impossible to make these movements in any but prearranged sequences, is called, in railroad vernacular, "interlocking," and in this sense the word will be used here. Interlocking has become a special art. The objects which it is sought to accomplish by interlocking, and the admirable way in which those objects are attained, may best be understood from an actual example. For that purpose we shall take a double-track junction completely equipped with signals, facing-point locks, and derailing switches (p. 205).
A general view of an interlocking frame was given on page 171 of this volume. Two levers from such a frame are here shown. The normal position of the levers is forward, as lever _A_. When pulled back, as lever _B_, the lever is said to be reversed.
Let it be supposed that a main-line train is to be passed eastward in the direction of the arrow _B_. The first movement of the signalman in the signal-tower would naturally be to lower signals 1 and 2. He attempts to pull over lever 1, but cannot move it, and, in spite of any effort or ingenuity on his part, that signal remains at danger. The reason is that lever 2 when normal locks lever 1 normal. The logic of this will be at once apparent. Clearing signal 1 is an indication to the engineer that the way is clear, and that he may pass the junction at speed. So long as this signal (which, it must be remembered, is a _caution_ signal) stands at danger he knows that he may pass it, but must be ready to stop before he reaches No. 2, the home-signal. Therefore No. 1 must never be lowered till all is arranged for passing the junction at speed. As the signalman cannot lower signal 1, he attempts to lower signal 2. Again he finds that he cannot budge the lever. It is locked by lever No. 3. This lever works a facing-point lock, which must be described just at this point (p. 206).
[Illustration: Diagram of a Double-track Junction with Interlocked Switches and Signals.