Urban rail transit includes: subway system, light rail system, urban rapid rail transit system, monorail system, tram system, automatic guided track (APM) system, maglev system. Urban rail transit is the backbone of urban public transportation, with the characteristics of energy conservation, land saving, large capacity, all-weather, pollution-free (or less polluting) and safety. It belongs to the green and environmentally friendly transportation system and is particularly suitable for large and medium-sized cities