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 7.8 km underground portion was run by Seoul Metro 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.