The Seoul Metropolitan Subway is one of the most heavily used rapid transit systems in the world, with well over 8 million trips daily on the system's ten lines totaling 287 km. The system serves Seoul, Incheon, Gyeonggi province and northern Chungnam province.
The Korail, the forerunner of the Seoul Metro, began operating Line 1 in 1974. Lines 2, 3, and 4 followed in the late 1970s and 1980s. The SMRT was formed in 1994 to take over operation of lines 5 to 8.
In the 1990's, Seoul has carried out probably the world's most ambitious subway extension program adding 160 km of new lines to the network.
This included the extension of former lines 2, 3 and 4 (15 km) and 4 totally new lines of which line 5 is 52 km long (including both eastern branches). New maps also include Incheon's first subway line which is linked to the Seoul subway network via Line 1.
Two new subway lines are currently under construction and have yet to open:
- Line 9 will run east from Gimpo Airport along the south bank of the Han River. Construction of the first phase began in April 2002 and is due for completion in 2008.
- New Bundang Line will be a new line from Gangnam in Seoul to Jeongja in Bundang, Seongnam. Construction is due to be completed in 2010.
Several existing lines are also currently undergoing extension:
- Line 3 will be extended south from Suseo to Garak Market and Ogeum, interchanging with lines 8 and 5 respectively, by 2009.
- By 2008, the Bundang Line will be extended north from Seolleung to Wangsimni and south to Yongin and Suwon, with eventual plans for it to link up with a new "Su-in" (Suwon-Incheon) line in two parts all the way to Incheon, completing, together with a section of Line 4, a southern outer semicircle.
- Line 7 will be extended west by 9.8km from Onsu to meet the Incheon Subway at Bupyeong-gu Office. Nine new stations will be added by 2010.
There are several major works still in the planning stage:
- A 10km extension to the New Bundang Line (see above) northward from Gangnam to Yongsan is under consideration. If approved, the project is estimated to cost at least 400 billion won.
- By the end of 2011, Seoul City plans to build a 10.72km 'light' subway line from Ui-dong to Sinseol Dong in northern Seoul. The line is expected to carry 110,000 passengers a day and will have 12 stations. It will connect to Line 4 at Sungshin Women's University, Line 6 at Bomun and Sinseol Dong (Lines 1 & 2).
- EverLine Rapid Transit System will be a new line branching off the Bundang Line extension and running east into Yongin and then to the theme park Everland.
- An extension of the Jungang Line from Deokso is also planned. This should eventually terminate at Yongmun.