There is a transfer to Line 1 at Sosa and Line 4 at Wongok.

​

All trains currently operate between Sosa and Wonsi.

​

The current line is 23.3 km (14.5 mi) long. The line is operated by Sosawonsi Operation Co., but the trains and track are supplied by Korail. Thus, all track is on the left-hand side.

​

All trains are stored at Line 4's Siheung Depot.

​

The second section will run from Sosa to Ilsan via Gimpo International Airport and Daegok. This section will share track with the Gyeongui-Jungang Line from Neunggok to Ilsan. Construction on this phase started on December 22, 2015 with a planned opening date of June 29, 2021.

​

Construction began on the third section on May 22, 2015. This section will connect Hongseong Station on the Janghang Line to Songsan. This section will not be run by metro trains, instead using the new EMU-250 trains. It would be connected to the rest of the line via track between Daegok and Hongseong. This section will open in 2022.

​

Tentative plans for the final section foresee the incorporation of the Gyooe Line, at present a single-track non-electrified line that runs from Neunggeok Station to Uijeongbu. The line currently sees very little use, with irregular freight service (All passenger services were discontinued in 2004). The line would have to be upgraded, and trains would divert at a junction before Daegok. It is planned for the combined Seohae Line to provide metro service in a northern semicircle around Seoul, complementing the southern semicircle formed by the Bundang and Suin Lines.