Seoul has a comprehensive urban railway network of 21 rapid transit, light metro and commuter lines that interconnects every district of the city and the surrounding areas of Incheon, Gyeonggi province, western Gangwon province, and northern Chungnam province. With more than 8 million passengers per day, the subway is one of the busiest subway systems in the world and the largest in the world, with a total track length of . In addition, in order to cope with the various modes of transport, Seoul's metropolitan government employs several mathematicians to coordinate the subway, bus, and traffic schedules into one timetable. The various lines are run by Korail, Seoul Metro, NeoTrans Co. Ltd., AREX, and Seoul Metro Line 9 Corporation.