East West Line

The first segment of what will be the East West line, between City Hall to Outram Park, was opened on 12 December 1987 as part of the North South line. It was extended westwards to Clementi on 12 March 1988. During the construction of the extension to Clementi, roads have to realigned for the new MRT line over Commonwealth Avenue West. The line later extended to Lakeside on 5 November 1988. The 15-kilometre (9.3 mi) eastern extension to Tanah Merah opened on 4 November 1989, and the opening ceremony was attended by then First Deputy Prime Minister Goh Chok Tong. At this point, the East West line was formed, reflecting the operational split from the North South line which has at the same time extended to Marina Bay, and uses a green coloured theme.

The 6-kilometre (3.7 mi) line extension to Pasir Ris station was opened ahead of schedule on 16 December 1989 with a sneak preview beforehand. Boon Lay station opened on 6 July 1990. Dover however, opened on the 18th of October, 2001. It opened between Buona Vista and Clementi, making it the first infill station of Singapore.

On 11 January 2011, the Tuas West extension, an extension of the East West line from Joo Koon to Tuas Link was announced. The extension has a span of 7.5 kilometres (4.7 mi), which spans a twin-tracked MRT viaduct, four above-ground stations and a 26-hectare depot to provide stabling and maintenance facilities for the additional trains that will be bought for the extension and to cater for future expansion of the line. The viaduct is integrated with part of a 4.8-kilometre-long (3.0 mi) road viaduct along Pioneer Road, which will increase the road capacity to cope with anticipated increase in traffic. The stations are Gul Circle, Tuas Crescent, Tuas West Road, and Tuas Link. On 4 May 2012, the Land Transport Authority marked the start of construction of the Tuas West extension with a groundbreaking ceremony at the site of the future Tuas Link.

The extension began service on 18 June 2017. During initial stages after opening, the westbound trains will alternate their terminus between Joo Koon and Tuas Link, where for every two trains, one train will terminate at the former, while the other train will terminate at the latter.