Seoul Metropolitan Subway Line 1 of the Seoul Metropolitan Subway is a rapid transit and commuter rail line which links central Seoul, South Korea to Soyosan Station in the northeast, Incheon in the southwest, and Sinchang (Soonchunhyang Univ.) via Suwon and Cheonan in the south. The central underground portion of this rail line is the oldest subway section in the Seoul Metropolitan Subway system. Its branches and services cover a large part of the Seoul Capital Area; totaling in route length. The underground section between Seoul Station and Cheongnyangni station, which is referred to as Seoul Metro Line 1 (), is currently operated by Seoul Metro. The line first opened in 1974 as the '''Korean National Railroad of Seoul''' with through services to national mainline railways from Seongbuk station (now: Kwangwoon University station) to Incheon and Suwon Stations. At the time, the underground portion run by Seoul Metro Corporation—one of Seoul Metro's predecessors—was called Line 1 and labeled red on maps. On the other hand, the remaining sections of the rail line run by Korail were labeled either blue or gray on maps, and the express lines were red. In 2000, the adjacent through-running Korail services from the Gyeongbu, Gyeongin and Gyeongwon Lines were officially bundled together as part of a greater Line 1, and the labeling on maps was changed to the current dark blue color. Frequent service is provided between Soyosan, Dongducheon, Uijeongbu, Cheongnyangni, Seoul, Yongsan, and Guro, where trains split between Incheon in the west and Byeongjeom and Cheonan in the south. Express trains operate from Yongsan and Seoul stations to Dongincheon and Sinchang(Soonchunhyang Univ.) stations. Trains run every 3-6 min in the central route between Seoul Station and Cheongnyangni, every 6-9 min between Seoul and Guro, every 8-10 min between Guro and Incheon and between Guro and Byeongjeom, every 10-15 min from Byeongjeom to Cheonan, and every 35 min between Cheonan and Sinchang and service up to the Dongducheon area and Soyosan station. Trains travel along Gyeongbu (Seoul-Cheonan), Gyeongin (Guro-Incheon), Janghang (Cheonan-Sinchang(Soonchunhyang Univ.)), and Gyeongwon (Hoegi-Soyosan) railway lines. The line runs on the left-hand side of the track, as opposed to the right-hand side of the track like all other Seoul Metropolitan Subway lines (another exception to this is Seoul Subway Line 4 although it runs on the right-hand side until Namtaeryeong station, and operates on the left-hand side for the parts afterwards) since railways in