At 41.9 kilometres (26.0 mi), the DTL is the longest underground and automated rapid transit line in Singapore as of 2017[update].

[4] Stage 3e, a two-station extension consisting of Xilin and Sungei Bedok, is under construction and is scheduled to begin operations in 2026.

[6] A two-station extension to connect with the North South Line at Sungei Kadut is also being planned and set to open in 2035.

[7] At the official opening of the Dover station on 23 October 2001, then-Minister for Communications and Information Technology Yeo Cheow Tong announced three MRT lines, two of which―the Bukit Timah Line and the northern part of the Eastern Region Line―are part of today's DTL.

[11] On 27 April 2007, then Transport Minister Raymond Lim announced that the Downtown Line would be built in three stages stretching 40 kilometres (25 miles) with 33 stations.

It began service on 22 December 2013,[19] with its official inauguration made on the day before by Prime Minister Lee Hsien Loong.

This was because one of the main contractors for the DTL2, Alpine Bau (which was building King Albert Park, Sixth Avenue and Tan Kah Kee), went bankrupt in mid-2013.

[33] On 28 June 2015, Transport Minister Lui Tuck Yew announced that the delay was "completely resolved" by the authorities and Stage 2's opening date was brought forward to 27 December 2015.

[34][35] The DTL 3 started operations on 21 October 2017, officially opened by Transport Minister Khaw Boon Wan.

[39] Also in February that year, the Land Transport Authority announced that the capacity of the depot will be expanded to stable 81 trains by 2019.

A study will also be conducted to determine the stops to be added between the Sungei Kadut and the existing Bukit Panjang stations.

This resulted in the section between Fort Canning and Mattar stations to be closed as the locomotive could not be moved until heat from its body dissipated.

After Geylang Bahru station, the line continues in a general eastwards direction, through the industrial areas of Kampong Ubi and Kaki Bukit (briefly paralleling Ubi Avenue 2 and Kaki Bukit Avenue 1), before continuing towards the Tampines Regional Centre and goes southwards, ending at Changi Business Park.

[58] LTA together with the operator, SBS Transit, conducted the necessary tests to ensure safety standards, functional performance and systems compatibility requirements were met before revenue service began on 22 December 2013.

[59] Testing for Stage 2 began on 25 October 2015 and rolling stock that was delivered to Gali Batu Depot commenced service in Stage 1 on 21 October 2015, while SBS Transit slowly moved the equipment managing the DTL trains to Gali Batu Depot during non-revenue hours.

[8] Kim Chuan Depot, together with the adjacent Tai Seng Facility Building, operated in a minor capacity until Stage 3 opened.

Platform screen doors (PSDs) along the line were installed by Westinghouse Signal (Invensys Rail Group), supplied by Faiveley.