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A crossover or" is a pair of switches that connects two parallel rail tracks, allowing a train on one track to cross over to the other. When two crossovers are present in opposite directions, usually one after the other, the four-switch configuration is called a double crossover. If the crossovers overlap in the shape of the letter X, it is dubbed a scissors crossover or diamond crossover in reference to the diamond crossing in the center.

On a crowded system, routine use of crossovers (or switches in general) will reduce throughput, as the switches must be changed for each train. For this reason, on some high-capacity rapid transit systems, crossovers between local and express tracks are not used during normal rush hour service, and service patterns are planned around use of the usually flying junctions at each end of the local-express line. In a setup where each of the two tracks normally carries trains of only one direction, a crossover can be used either to detour ("wrong-rail") around an obstruction or to reverse direction. A crossover can also join two tracks of the same direction, possibly a pair of local and express tracks, and allow trains to switch from one to the other.

An example of a typical single crossover.

An exicornt: two pairs of switches linking two tracks to each other in both directions in the formation of an X.

Defunct diamond crossover.jpg A scissors crossover on a defunct railway.