The Downtown line (DTL) is a medium-capacity Mass Rapid Transit line in Singapore. It runs from Bukit Panjang in the north-west to Expo in the east. Currently, the line is 41.9 kilometres (26.0 mi) long and has 34 stations, all of which are underground. The line is coloured blue on the rail map.
The original line was built in 3 stages, which opened in 2013, 2015 and 2017 respectively.  Stage 3e, an extension of the line to Sungei Bedok, is to be completed in 2024. The line connects the North-Western and Central-Eastern regions to the downtown of Singapore.
This line is the fifth MRT line on the network to be opened, the third to be entirely underground and, with 3-car trains, the second medium-capacity line after the Circle line. When fully completed in 2024, the line will be about 44 kilometres (27 mi) long with 36 stations and will serve more than half a million commuters daily. It will also be the longest rapid transit line in Singapore to use completely automated, driverless trains, and among the longest of such lines in the world. Travelling from one end to the other will take around 67 minutes. It is the second MRT line to be operated by SBS Transit after the North East Line.
The line begins in Bukit Panjang, and then goes through the ‘school district’ and food enclaves of Bukit Timah before entering the central area, passing through Little India before reaching the Central Business District via Bugis and Chinatown. After this, it goes to Fort Canning, Geylang Bahru and MacPherson after intersecting itself.
On 14 June 2005, the Land Transport Authority announced the Downtown Extension of the Circle line to serve the Downtown at Marina Bay area, where an integrated resort (Marina Bay Sands) and Singapore's second botanical garden (Gardens by the Bay) was to be located. The 3.4-kilometre fully underground line was estimated to cost S$1.4 billion. Construction of the extension began in January 2008. During the construction of the North East line at Chinatown station, platform provisions were built to facilitate an interchange station. In August 2011, SBS Transit won the bid and was appointed to be the operator of the Downtown Line, under a new framework which would see the authority remaining the owner of the line.
The original line was built in 3 stages, as follows:
Stage 2 of the Downtown Line was first conceptualized and announced as the Bukit Timah Line. It was supposedly a 20-kilometre line that would connect the new downtown with the Bukit Panjang, Upper Bukit Timah and Bukit Timah corridor, alleviating the heavy traffic travelling along the sector, but a full route was released. Provisions were provided at Nicoll Highway station which would have seen the line terminating there and interchanging with the Circle line. However, the Nicoll Highway collapse and subsequent feasibility works and deemed the old station unusable. The stage is 16.6-kilometre (10.3 mi) long with 12 stations connecting Bukit Panjang and Rochor stations, including four interchange stations. Construction for Stage 2 began on 3 July 2009 with a groundbreaking ceremony at Beauty World station.
In October 2014, it was announced that Stage 2's opening would be pushed back to the first quarter of 2016. It had to be delayed when main contractor Alpine Bau went bankrupt in mid-2013. 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 reverted to 27 December 2015.
Stage 3 of the Downtown Line goes from Chinatown to Expo. The station locations and finalized route were unveiled on 20 August 2010. Stage 3 of the Downtown line is 21-kilometre (13 mi) long and serve 16 stations. It started operations on 21 October 2017, officially opened by the Transport Minister Khaw Boon Wan.
To serve the line, the Gali Batu Depot was opened in December 2015 to stable 42 trains. It is situated at part of the former Kwong Hou Sua Teochew Cemetery off Woodlands Road. 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.
Final costs of building the line may reach an estimated about S$20.7 billion, up more than 70% from an initial estimation of S$12 billion. In January 2013, plans for a southern extension which will run from Expo through the East Coast area, interchanging with the Eastern Region line were announced for completion by 2025. On 31 May 2017, LTA announced that Stage 3 of the Downtown Line commenced operations on 21 October 2017.
DTL3e was announced on 15 August 2014, in conjunction with the announcement of the Thomson-East Coast MRT line. Two stations, Xilin MRT station and Sungei Bedok MRT station will be added to the Downtown Line, with Sungei Bedok as an interchange station with the Thomson–East Coast line. It will add an additional 2.2 km to the line with the extension. Due in 2024, Stage 3e will join the current East West and future Thomson–East Coast lines that runs through Marine Parade.
A malfunction in the platform screen doors at Botanic Gardens MRT station resulted in trains bypassing the station and a seven-hour disruption on Wednesday (3 May 2017). Downtown Line operator SBS Transit said that the doors malfunctioned at 5.45am due to a signalling-related fault, and could not open automatically. The fault was rectified at 12.54pm and the platform opened for passenger service, with the affected door remaining closed for repairs. 
Downtown MRT Line
|Station number||Station name||Image||Interchange and notes|
|Bukit Panjang||Interchange with the Bukit Panjang LRT|
|N/A||Located at Hume Avenue; Shell station; Not In Operation|
|King Albert Park|
|Tan Kah Kee|
|Botanic Gardens||Interchange with the Circle line|
|Stevens||Interchange with the Thomson–East Coast line (2021)|
|Newton||Interchange with the North South line|
|Little India||Interchange with the North East line|
|Bugis||Interchange with the East West line|
|Promenade||Interchange with the Circle line|
|Bayfront||Cross-platform interchange with the Circle line extension|
|Chinatown||Interchange with the North East line|
|MacPherson||Interchange with the Circle line|
|Tampines||Interchange with the East West line|
|Expo||Interchange with the East West line's Changi Airport Branch Line|
|Downtown Line Extension (In 2024)|
|Sungei Bedok||Interchange with the Thomson-East Coast line (2024)|
The station code "DT4" is reserved for future use.
Currently, the Downtown Line has one type of rolling stock, the Bombardier MOVIA C951 cars, running in a three-car formation. They are stabled at Gali Batu Depot after it opened with Stage 2 of Downtown Line on 27 December 2015. For the period between the operation of Downtown Line Stage 1 and Downtown Line Stage 2, trains were stabled at a maintenance facility that was built at Marina Bay as part of the Circle line project. Kim Chuan Depot housed the Operations Control Centre for the Downtown Line Stage 1 until Gali Batu Depot was ready to be used.
On 12 October 2012, the first of 11 trains for the Downtown Line Stage 1 arrived at Jurong Port. It was transported to Kim Chuan Depot to undergo testing by LTA before it was handed over to SBS Transit. As of 28 February 2013[update], Bombardier had delivered five of the 11 trains for Downtown Line Stage 1. 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.
Testing on the Downtown Line Stage 2 began on 25 October 2015 and rolling stock that was delivered to Gali Batu Depot have commenced service on Stage 1 starting on 21 October 2015. Kim Chuan Depot, together with the adjacent Tai Seng Facility Building will operate on a minor capacity until the Downtown Line Stage 3 opens. A new depot, named the East Coast Integrated Depot will provide additional stabling capacity to the line in 2024.
Testing on the Downtown Line Stage 3 began on 15 August 2017, after the late opening of Downtown Line from 27 May 2017, exactly two months before Stage 3's Open House, themed "Rail Of Fun". Most of the other rolling stock that had not carried passengers before and were stabled in Gali Batu Depot as early as 2015 finally entered revenue service progressively from that date onwards. Testing on DTL Stage 3 will be restricted from Fort Canning to Expo from 19 September 2017.
The Downtown Line is equipped with Siemens (previously Invensys before Siemens acquisition) Trainguard Sirius Communications-based train control (CBTC) moving block signalling system with Automatic train control (ATC) under Automatic train operation (ATO) GoA 4 (UTO). The subsystems consist of Automatic train protection (ATP) to govern train speed, Controlguide Rail 9000 Automatic Train Supervision (ATS) to track and schedule trains and Trackguard Westrace MK2 Computer-based interlocking (CBI) system that prevents incorrect signal and track points to be set.
A fall-back signalling system, relying on conventional track-circuit occupancy detection, is included to ensure fully automatic operation and train protection independent of the radio system.