Short tracks joining two neighbouring, mostly parallel tracks on rail lines with more than one track.
Mapping
This tag is broadly intended to be used for crossovers that connect two or more main tracks (i.e. without service=*). For crossover tracks that connect to a yard, siding or spur track on one or both ends, they should be tagged service=yard, service=siding, or service=spur accordingly.
Many renderers, including OpenStreetMap Carto, display tracks with service=crossover just as prominently as main tracks. Others, like OpenStreetMap Americana, give this tag a unique treatment. In either case, minor crossovers tend to stick out when tagged this way, and should generally be tagged to match the type of service track that they connect to.