Mention was made, in connection with the lever, of points. Before going further we will glance at the action of these devices for enabling a train to run from one set of rails to another. Figs. 98 and 99 show the points at a simple junction. It will be noticed that the rails of the line to the left of the points are continued as the outer rails of the main and branch lines. The inner rails come to a sharp V-point, and to the left of this are the two short rails which, by means of shifting portions, decide the direction of a train's travel. In Fig. 98 the main line is open; in Fig. 99, the branch. The shifting parts are kept properly spaced by cross bars (or tie-rods), A A.
Fig. 98.—Points open to main line.
Fig. 99.—Points open to branch line.
It might be thought that the wheels would bump badly when they reach the point B, where there is a gap. This is prevented, however, by the bent ends E E (Fig. 98), on which the tread of the wheel rests until it has reached some distance along the point of V. The safety rails S R keep the outer wheel up against its rail until the V has been passed.
POINTS AND SIGNALS IN COMBINATION.
Let us suppose that a train is approaching the junction shown in Figs. 98 and 99 from the left. It is not enough that the driver should know that the tracks are clear. He must also be assured that the track, main or branch, as the case may be, along which he has to go, is open; and on the other hand, if he were approaching from the right, he would want to be certain that no train on the other line was converging on his. Danger is avoided and assurance given by interlocking the points and signals. To the left of the junction the home and distant signals are doubled, there being two semaphore arms on each post. These are interlocked with the points in such a manner that the signals referring to either line can be pulled off only when the points are set to open the way to that line. Moreover, before any shifting of points can be made, the signals behind must be put to danger. The convergence of trains is prevented by interlocking, which renders it impossible to have both sets of distant and home signals at "All right" simultaneously.
WORKING OF BLOCK SYSTEM.