Throw-off or _trap points_, are introduced to throw an engine or train off the rails of a siding on to the ballast, and so avoid a collision with any other train which may be standing or passing on the line of rails with which such siding forms a connection. Fig. 484 is a diagram 321 sketch of the arrangement, in which the main-line points are indicated by the letter A, and the trap points by the letter B; one series of rodding actuated by one lever in the signal-cabin works both the main-line points and the trap points at the same time and by the same movement. The connections are so made that when the points A are set for the passage of trains on the main line, the trap points B are set open to throw off on to the ballast, as shown in Fig. 484; and when the main-line points A are set to allow a train to pass from the siding on to the main line, the trap points B are closed, as shown on Fig. 485. A disc or other signal, worked or interlocked with the points, is placed near B to notify the engine-driver when he may pass out of the siding on to the main line; but should he from any cause proceed before the points are properly set and the corresponding signal given, his engine would run off at the ends of the rails C, C, and be derailed on to the ballast. The inconvenience caused by such derailment would be trifling compared with what might result from a collision with a train standing or passing on the main line. In some cases the siding is continued onwards for a considerable distance from the trap-point rails C, C, as indicated by the dotted lines D D, and terminates with a dead end. When this arrangement can be adopted, derailment is obviated, and the engine is brought to a stand by a buffer-stop at the end of siding. On no account should trap points be placed close to the top edge of a high embankment, or up to the abutment or wing walls of an under-line bridge, where an engine running through them accidentally might fall down a considerable height, and cause serious results. All sidings joining on to main lines should be trapped as above described, and when properly signalled and interlocked in the signal-cabin, the traffic-working can be carried on with increased facility and security.