The Seoul Metropolitan Subway is a metropolitan railway system consisting of 23 rapid transit, light metro, commuter rail and people mover lines located in northwest South Korea. The system serves most of the Seoul Metropolitan Area including the Incheon metropolis and satellite cities in Gyeonggi province.
Some regional lines in the network stretch out to rural areas in northern Chungnam province and western Gangwon province, that lie over 100 km away from the capital, as well as Suwon.
The network consists of numbered lines 1–9, which serve Seoul City proper and its surroundings and other specifically named regional railways that serve the greater metropolitan region and beyond.
Seoul Metropolitan Subway has a total length of lines reaching 1,182.7 km (734.9 mi), making it become the world’s longest multi-operator metro system by total route length.
Most of the system is operated by three companies – Seoul Metro, Korail (Korea National Railroad) and Metro 9. Its first metro line, Line 1, started construction in 1971 and opened in 1974, with through-operation to Korail suburban railways.
Today, the network is one of the largest and most efficient urban railway systems in the world, with 331.5 km (206.0 mi) of track on lines 1–9 alone; wireless and internet service on all trains; and platform screen doors at almost all stations.
According to en.wikipedia
