When it opened in 2009, Kim Chuan train depot was hailed as the biggest underground depot in the world. By 2025 it will be even bigger.
The 12ha facility, which stables trains for the Circle Line (CCL), will be expanded by another 16ha, allowing it to hold 133 trains, up from the current 70. The project will cost $2.3 billion.
This will give the Land Transport Authority (LTA) the "ability to expand the (train) fleet" in tandem with expected passenger growth on the CCL, which will become a complete loop by 2025, said chief executive Chew Men Leong.
To optimise land use, the expanded section of the Kim Chuan depot will also have an above-ground facility to house 550 buses.
Yesterday, Senior Minister of State for Transport Ng Chee Meng announced the alignment and station locations for the sixth stage of the CCL (CCL6) - a three-station, 4km link joining the existing HarbourFront and Marina Bay stations.
More than 400,000 commuters use the Circle Line every day.
Mr Chew said: "With the closure of the Circle Line (loop), this will certainly expand. We don't have a full estimate... It also depends on the developments that will come as we complete the CCL6, that will create a new catchment and, therefore, new ridership."
Besides offering commuters direct routes to the Central Business District (CBD) and Marina Bay area, the new link will also extend the rail network into areas, such as Spottiswoode, Everton Park and the southern edge of the CBD.
There are currently 47 trains in the Circle Line fleet and another 17 will be added by the end of next year. Mr Chew said the LTA will be adding even more trains after that to meet rising demand.
An LTA spokesman told The Straits Times: "CCL6 was not built earlier as the catchment and ridership for that section of the line was projected to be insufficient for the additional stations to be viable."
With expected development and travel demand growth along the CCL corridor in areas such as Buona Vista, one-North, Paya Lebar Sub-Regional Centre and the CBD extension in Shenton Way and Marina Bay, there was now a greater need to support east-west connections on the CCL southern corridor by closing the loop.
•Additional reporting by Christopher Tan