Seoul Subway Line 4 of the Seoul Metropolitan Subway is a long line crossing from the southwest to the northeast across the Seoul Metropolitan 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; the newly built northern section (officially called the Jinjeop Line) is correspondingly owned by a company owned by Namyangju-si, with operations contracted to Seoul Metro. In 2022, the Seoul Metro operated section (excluding the Jinjeop Line) had an annual ridership of 219,587,000 or about 601,608 passengers per day.

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.

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