331.—Switches and Crossingsdetails

Switches and crossings, or, as they are more commonly termed, points and crossings, are used for the purpose of allowing the trains to pass or cross over from one line of rails to the other. Several different methods have been devised for doing this. One of the simplest plans, and that most frequently adopted, is to lay down a short line of rails connecting the other two, and thus establishing the desired communication. It is, however, necessary to have ready and expeditious means of connecting and disconnecting this short line with the main line, according as it is intended that the trains shall leave or continue upon the latter; this is effected by the contrivance termed a switch, which is shown in our figure.

a b and c d are portions of the rail of the main line, and e f and g l portions of the short line branching from it. All these parts are immovably fixed in the ordinary manner, with the exception of the two rails f i and k l. These, which are termed the tongues of the switch or points, are only fixed at one of their ends f and k, on which they turn as centres; the other ends are tapered away to nearly a point, a slight recess being sometimes cut in the other lines, as at i and l, into which they fit. These tongues are connected together by a bar m n o, by means of which they are preserved at such a distance apart, that when either tongue is in contact with the rail near it, the other shall be removed from the one opposite a sufficient space to allow the engine or carriage wheels to pass between. (Suppose the train to come in the direction of the arrow.) In order to keep the train on the main line, or to leave the same and enter the branch line, it only becomes necessary to move the bar m n oo. When m n o, or the bar which moves the switch, is in the position as shown at A, the carriages will leave the main line; but if shifted into the position shown at B, then they will continue on their course along the main line.

It will assist the student to understand what has been said, if he will consider that the flange of the wheel bears against the inside of the rail. It is usual to have the points so arranged that they are kept in the position shown at B (where the main line is not interrupted) by a self-acting weight, the attendance of a pointsman being necessary to move them into the position A, if it is desirable that the train should go off the main line. Two guard rails, p q and r s, are employed to prevent the flanges of the wheels from striking against the point where the two lines intersect each other at t.

(p. 301)