Railroad switch

A railroad switch (known in British English as a point or a set of points) is a construction where a railroad track A splits into two tracks B and C. It can be in two positions, which determine whether a train coming from A is led to B or C. A train coming from B or C is led to A anyway, but this movement is smoother if the switch is set to the appropriate position.

The switch contains a pair of linked tapering rails that can be moved laterally (point blades).

The position of the switch is usually changed electrically and controlled from a remote control center or signal box, from where staff also alter semaphores or light signals correspondingly. In rarely used sidings, self-contained marshalling yards or on heritage railways a switch might be manually operated with a points lever. A switch point of a tram is often operated remotely by the driver.

With a right switch A and B form a straight track and C is to the right of B, with a left switch C is to the left. A switch may also be symmetric, or tracks AB and AC may both be curved in the same direction.

A double switch or English switch is a crossing of railroads AB and CD at a small angle, with the possibility to pass from one to the other (go from A to D and from B to C). This construction is equivalent to one crossing and four switches (two left and two right), but without the need to be able to set all four independently: the whole double switch has only two positions: crossing and bending.

The correct setting of points is fundamental to the safe running of a railway. A fatal train accident at Potters Bar[?], Hertfordshire, UK occurred in May 2002, when a switch sprang to a different position as a coach crossed it. The front coach wheels therefore progressed from A to B whereas the rear wheels slewed towards C, causing the whole coach to detach from the train and slew sideways across the platform ahead. Thankfully the movement of the switch occurred beneath the final coach, so that although seven people were killed, the front coaches were spared. Poor maintenance of the points was held to be the primary cause of the crash.

Monorail systems have special switches, see the external link (http://www.monorails.org/tMspages/switch.html), a page which includes animations.