SML9 contracted the operation to Seoul Line9 (SL9), a joint venture of Hyundai Rotem (20%) and RATP Dev Transdev Asia (80%, formerly Veolia Transport Korea).

In 2001, the city passed the feasibility study and execution designs for line 9 and confirmed the commencement of construction.

Due to increasing ridership (numbers soon exceeded 250,000 per day), 12 additional four-car trains were ordered to alleviate capacity constraints.

On September 30, 2013, additional express trains were added during commuting times of 7 to 9 AM (from 22 to 36), further reducing the headway.

[7] Phase 3 further extended the line to VHS Medical Center Station in eastern Seoul,[8] and was opened on December 1, 2018.

[9] The following workdays saw a 6.4% increase in passengers, roughly 145,000 people, as the line now services a larger area to the east.

Prior to opening of the line, the operator requested a base rate of 1,300 won.

The city government decided against this by citing a domestic financial crisis and potential complaints from riders if the price was set at a higher rate than the rest of the subway system.

Seoul City will be in negotiations again with the help of legal counsel to determine the differences of opinions in the original agreement.

[19] On November 20, 2008, the Seoul Metropolitan government announced plans to build Phase 3 between October 2010 and December 2015.