Seoul Subway Line 4

Seoul Subway Line 4 (dubbed The Light 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; 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.

History
1985:
 * 20 April: Line 4 is officially opened from Sanggye to Sadang.

1993:
 * 21 April: The line is extended northward from Sanggye to Danggogae, and southward from Sadang to Namtaeryeong.

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 Jinjeop Line, a northward extension of Line 4, opens from Danggogae to Jinjeop.

Future
Express services are planned to start skipping various stations north of Sanbon station by 2023 to cut travel times.

Depots, junctions, and points of interest
(from Jinjeop to Oido)
 * Jinjeop Depot (opening December 2024, will replace Changdong Depot for Seoul Metro-operated rolling stock)
 * Turnback siding (underground) after Danggogae Station
 * Changdong Depot (used for Seoul Metro class 4000, with non-revenue connection to Line 1)
 * Non-revenue connecting track to Line 3 before Chungmuro Station
 * Dongjak Bridge
 * Chongshin Univ. – Sadang scissors crossover
 * Hanyang University, ERICA Campus
 * Turnback siding (underground) after Sadang Station
 * Namtaeryong–Seonbawi track crossing point (flying crossover, switch from right to left-hand traffic, or vice versa)
 * The voltage/current switches between DC 1,500 V ↔ AC 25,000 V
 * Non-revenue crossover to Line 1 after Geumjeong Station
 * Non-revenue connecting track to Seohae Line before Choji Station
 * Ansan Depot (used for simple maintenance of Korail Class 341000 train)
 * Siheung Depot (used for Korail Class 341000 maintenance and also for heavy maintenance of Korail Class 311000 trains operated on Line 1)
 * The largest scale of shell mounds in the South Korean west coast in Oido

Seoul Metro

 * Seoul Metro 4000 series
 * 1st generation, DC only: 4-01~4-26 (Restricted to Jinjeop-Sadang)(Retirement in Progress)
 * 1st generation, DC & AC: 4-51~4-71 (Retirement in Progress)
 * 2nd generation: 4-81~4-85
 * 3rd generation: 4-50~4-70
 * 4th generation: 4-01~4-26

Korail

 * Korail Class 341000 (ex-Korail Class 2030)
 * 2nd generation: 341-26~341-30
 * 3rd generation: 341-31~341-37
 * 4th generation: 341-38~341-60
 * Trains 341-53~341-55 are temporary running on Line 1.

Seoul Metro

 * Seoul Metro 3000 series (former 4000 series)
 * Wide-width GEC Traction chopper resistor controlled electric car (1985–1993; transferred to Seoul Subway Line 3)

Korail

 * Korail Class 1000 (Ansan Line only, transferred to Seoul Subway Line 1)
 * Korail Class 341000 1st generation: 341-01~341-25, retired in 2024