Fig. 45.—Conical Wheels.
Fig. 46.—Centrifugal Force.
The wheels of railway carriages and engines differ from those of ordinary carriages in being fastened in pairs upon the axles, with which they revolve (see Fig. [45]). The tire of the wheel is conical, the slope being about 1 in 20; that is, in a wheel 5 in. broad the radius of the outer edge is ¼ in. less than that of the inner; and the rails are placed sloping a little inwards. The effect of this conical figure is to counteract any tendency to roll off the rails; for if a pair of wheels were shifted a little to one side, the parts of the tires rolling upon the rails being then of unequal circumference, would cause the wheels to roll towards the other side. The conical shape produces this kind of adjustment so well that the flanges do not in general touch the rails. They act, however, as safeguards in passing over curves and junctions. In curves the outer line of rails is laid higher than the inner, so that in passing over them the train leans slightly inwards, in order to counteract what is called the centrifugal force, to which any body moving in a curve is subject. This so-called force is merely the result of that tendency which every moving body has to continue its motion in a straight line. A very good example of the effect of this may be seen when a circus horse is going rapidly round the ring. The inclination inwards is still more perceptible when a rider is standing on the horse’s back, as shown in Fig. [46]. The earth’s attraction of gravity is pulling the performer straight down, and the centrifugal force would of itself throw her outwards horizontally. The resultant or combined effect of both acts is seen in the exact direction in which she is leaning, and it presses her feet on the horse’s back, the animal itself being under similar conditions. It is obvious that the amount of centrifugal force, and therefore of inward slope, will increase with the speed and sharpness of the curve, and on the railways the rails are placed so that the slope counteracts the centrifugal force when the train travels at about the rate of twenty miles per hour.
Fig. 47.—Points.
A very important part of the mechanism of a railway is the mode of passing trains from one line of rails to another. Engines and single carriages are sometimes transferred by means of turn-tables, but the more general plan is by switches, which are commonly constructed as shown in Fig. [47]. There are two rails, A and B, tapering to a point and fixed at the other end, so that they have sufficient freedom to turn horizontally. A train passing in the direction shown by the arrow would continue on the main line, if the points are placed as represented; but if they be moved so that the long tongue is brought into contact with the rail of the main line, then the train would run on to the side rails. These points are worked by means of a lever attached to the rod, C, the lever being either placed near the rails, or in a signal-box, where a man is stationed, whose sole duty it is to attend to the points and to the signals. The interior of a signal-box near an important junction or station is shown in Fig. [48], and we see here the numerous levers for working the points and the signals, each of these having a connection, by rods or wires, with the corresponding point or signal-post. The electric telegraph is now an important agent in railway signalling, and in a signal-box we may see the bells and instruments which inform the pointsman whether a certain section of the line is “blocked” or “clear.” The signals now generally used on British railways are made by the semaphore, which is simply a post from which an arm can be made to project. When the driver of the train sees the arm projecting from the left-hand side of the post, it is an intimation to him that he must stop his train; when the arm is dropped half-way, so as to project 45° from the post, it is meant that he must proceed cautiously; when the arm is down the line is clear. These signals, of course, are not capable of being seen at night, when their place is supplied with lamps, provided with coloured glasses—red and green—and also with an uncoloured glass. The lamp may have the different glasses on three different sides, and be turned round so as to present the required colour; or it may be made to do so without turning, if provided with a frame having red and green glasses, which can be moved like spectacles in front of it. The meanings of the various coloured lights and the corresponding semaphore signals are these:
| White | All right | Go on. | |
| Green | Caution | Proceed slowly. | |
| Red | Danger | Stop. |
Fig. 48.—Signal-box on the North London Railway.