Seoul Metropolitan Subway Line 4 (dubbed The Blue Line) of the Seoul Metropolitan Subway is a long line crossing from the southwest to the northeast across the Seoul National Capital Area. The central section in Seoul City is operated by Seoul Metro with some trains offering through service to Korail's Ansan and Gwacheon Lines. The southern terminus (Oido) is in Jeongwang 4-dong, Siheung City, and the northern terminus (Jinjeop) is in Jinjeop-eup, Namyangju-si, Gyeonggi-do. In 2022, the Seoul Metro operated section had an annual ridership of 219,587,000 or about 601,608 passengers per day. Northbound trains that run on the Ansan and Gwacheon Lines terminate at Danggogae, except during night time where they short-turn at various stations. Southbound trains that run on the Ansan and Gwacheon Lines terminate at Oido or Ansan. All trains from Jinjeop short-turn at Sadang, though some short-turn trains to Sadang start at Danggogae or terminate one station south at Namtaeryeong. Express train service stops at all stations between Danggogae and Sanbon, then at Sangnoksu, Jungang, Choji, Ansan, Jeongwang, and Oido. The express service only operates during rush hours on weekdays. History 1985: 20 April: Line 4 is officially opened from Sanggye to Samseon-gyo. 1993: 21 April: The line is extended northward from Sanggye to Danggogae. 1994: 1 April: The line is extended southward from Sadang to Ansan when a section of the Gwacheon Line (from Seonbawi to Indeogwon) and Namtaeryeong Station open. 2000: 28 July: The line is extended westward from Ansan to Oido. 2003 18 July: Surisan Station opens as an in-fill station on the Ansan Line section. 2010 Ansan Line AM express service is launched in the northbound direction only (starting from Ansan). Trains stopped at Jungang, Sangnoksu, and Sanbon, before continuing local up to Danggogae. 2014 1 September: Southbound PM express service is launched (terminating at Ansan). Trains make the same stops as their AM express service counterparts. 2017 7 July: Express service is extended to Oido. In addition to the stops they made before, trains make all stops between Ansan and Oido. 2020 12 September: Express service is modified, with trains now stopping at Choji but skipping Singiloncheon. 2022: 19 March: The line is extended northward from Danggogae to Jinjeop. Future Express services are planned to start skipping various stations north of Sanbon station by 2023 to cut travel times. Stations Depots, junctions, and points of interest (from Danggogae to Oido) Turnback