Many rapid transit systems run relatively static routings, so that a train "line" is more or less synonymous with a train "route". In New York City, routings change often, for various reasons. Within the nomenclature of the subway, the "line" describes the physical railroad track or series of tracks that a train "route" uses on its way from one terminal to another. "Routes" (also called "services") are distinguished by a letter or a number and "Lines" have names. Trains display their route designation.