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]).
Diagram of a Double-track Junction with Interlocked Switches and Signals.
A is the west-bound main line track; B, the east-bound; C and D are the west-bound and east-bound branch-tracks. Nos. 1, 10, and 12 are distant signals; Nos. 2, 9, and 11, home signals; Nos. 3, 6, and 8, facing-point locks; and Nos. 4, 5, and 7 are switches. The levers which move all of these parts are placed side by side in a frame in the signal-tower. It will be noticed that No. 7 is a switch designed merely to derail an engine on track A. A similar switch is provided on track C, and is worked by the same lever which works junction switch No. 5. In the sketch all levers are supposed to stand in their "normal" position, all signals are at danger, and the switches are set for the main line. The switches themselves are not locked in this position of the facing-point lock levers.
The front rod of the switch, that is, the rod which connects the points of the two moving rails of the switch, is pierced with two holes placed a distance apart just equal to the throw of the switch. In front of these holes is a bolt which is worked by a lever in the signal-tower. After the switch is set the lock-lever is reversed and the bolt enters one of the holes, thus securely locking the switch in position. There is one other interesting feature of this facing-point lock. It has happened very often that a switch has been thrown under a moving train, splitting the train and derailing more or less of it. This class of accidents is especially likely to happen when train movements are very frequent, and may be prevented by the use of the "detector-bar." This is a bar about forty feet long, placed alongside the rail, and carried on swinging links, like those of a parallel ruler, in such a way that any effort to move the bar lengthwise of the rail must raise it above the top of the rail. This bar is moved by the same lever which moves the locking-bolt. So long as there is a wheel on the rail above the detector-bar it cannot be moved, therefore the locking-bolt cannot be withdrawn, and the switch cannot be moved until the train has passed completely off it.
Split Switches with Facing-point Locks and Detector-bars.
(The rod on the right of the track is the mechanical connection to the lever in the signal-tower by which the locks and detector-bars are moved.)