The MRT Blue Line () or MRT Chaloem Ratchamongkon Line () is Bangkok's third rapid transit line, following Sukhumvit Line and Silom Line of the BTS Skytrain. It is the first line of the MRT system and is operated by Bangkok Expressway and Metro (BEM). The line has a total length of , operating as a semi-circle or spiral route from Lak Song to Tha Phra which also serves as a self-interchange station.

The first stage of the MRT Blue Line, a semi-circle alignment from Hua Lamphong to Bang Sue opened on 3 July 2004, which was then followed by a extension to Tao Poon opened on 11 August 2017. The line was extended to the west from Hua Lamphong through new stations in the Chinatown and Rattanakosin Island on 29 September 2019. Finally, the line was extended south from Tao Poon to Tha Phra through new stations in Thonburi on 30 March 2020, completing the semi-circle line. Another 7.8 km (4.8 mi), 4 station extension from Lak Song to Phutthamonthon Sai 4 is planned but remains unbuilt.

The MRT Blue line connects major business, residential and cultural areas of Bangkok. In late 2019, the average daily ridership was 400,000.https://www.khaosod.co.th/economics/news_2933591. Khaosod, 30 September 2019 During the third wave of COVID-19 in Bangkok, ridership fell to between 76,000 - 116,000 per weekday during the year 2021.

Route alignment The MRT Blue Line begins from the upper platform of Tha Phra station. It runs on an elevated viaduct northward along Charan Sanit Wong Road to Bang O station before crossing the Chao Phraya River, entering Bang Pho station and Tao Poon station in Bang Sue District. The line then dives underground to reach Bang Sue. It follows the Kamphaeng Phet, Phahon Yothin and Lat Phrao Roads, then turns south following Ratchadaphisek Road to Queen Sirikit National Convention Centre station. It then turns west following Rama IV Road. It passes Hua Lamphong station in Pathum Wan District, through Chinatown, then under the Chao Phraya River before ascending onto an elevated viaduct to the lower platform of Tha Phra station. The line continues west along Phet Kasem Road to Lak Song station in Bang Khae, situated near Kanchanaphisek Road or Western Outer Ring road. It forms a quasi circle loop around Bangkok.

History

Officially named Chaloem Ratchamongkhon (Thai สายเฉลิมรัชมงคล) – "Celebration of Royal Auspice" – or informally but commonly called as the "MRT Blue Line", this was the first metro line under the newly formed government agency, the Mass Rapid Transit Authority of Thailand (MRTA). Most civil infrastructure was provided by this government agency and handed over to a private sector on a 25-year concessionaire agreement. The winning bidder was Bangkok Expressway and Metro (BEM). Under this agreement, BEM provides maintenance and engineering equipment, including electrical trains, signalling systems, SCADA, communication, platform screen doors and fully operate the system from Hua Lamphong to Bang Sue. BEM has subcontracted maintenance of the system for 10 years to Siemens and seven-year maintenance contracts to two local maintenance services.

In 1996, the Overseas Economic Cooperation Fund (OECF), later known as the Japan Bank for International Cooperation (JBIC), granted the loans for the project with an official development assistance (ODA) scheme. Construction of the line began on 19 November of that year, when Crown Prince His Royal Highness Crown Prince Maha Vajiralongkorn (later King Vajiralongkorn) came to lay the foundation stone for the construction of the MRT project, which is Thailand's first subway project, with the aim of relieving Bangkok's traffic problems. The first contract was awarded to a joint venture between CH. Karnchang Public Company Limited, Bilfinger-Berger, Kumagai Gumi, and Tokyu Construction Co. for the southern section, while one of the remaining contractors, the northern section, was awarded to ION Joint Venture, which comprises Italian Thai Development, Obayashi Corporation, and Nishimatsu Construction, and the depot was awarded to Siam Nippon Metro Consortium, composed of Hazama Corporation, Maeda Corporation, Mitsui & Co., Kajima, T.S.B. Trading, and Siam Syntech Construction Public Co., Ltd. Tunneling works began on 5 February 1999.

During construction, the consultancy was led by the "Berger-CSC 1 Consortium," comprising the following consulting firms: Louis Berger and many more, as well as a joint venture of Mott Macdonald and De Leuw Cather.

The project suffered multiple delays not only because of the 1997 economic crisis, but also due to challenging civil engineering works of constructing massive underground structures deep in the water-logged soil upon which the city is built. The MRT Blue Line opened for a limited public trial period of several weeks starting on 13 April 2004. On 3 July 2004, the line was officially opened at 19:19 local time by HM King Bhumibol and Queen Sirikit who were accompanied by other members of the royal family. Within 30 minutes of its opening, sightseers filled the system to its maximum capacity, but after the initial rush ridership settled down to around 180,000 riders daily—considerably lower than projections of over 400,000, despite fares being slashed in half from 12 to 38 baht to 10-15 baht per trip.Bangkok's First Underground Metro Opens International Railway Journal July 2004 page 32 It was scheduled to be opened in 2003, in time for the same year's APEC summit, and later pushed back again to April and then August of the following year. However, no problems were encountered during the test run; the official opening was brought forward.

During the early years, it was named the subway as an alternate name.

Extension to Tao Poon As part of the rapid transit master plan the original MRT Blue Line section was planned to be extended from Hua Lamphong to Lak Song, and from Bang Sue to Tha Phra, forming a circle open-loop route. The contract for the first part of extension to Tao Poon was signed on 26 August 2009 as part of MRT Purple Line contract as it provided interchange to the MRT Purple Line. The Bang Sue to Tao Poon extension opened on 11 August 2017. This added the first elevated station, Tao Poon, to the Blue line.

Extensions to Lak Song and Tha Phra The major extension of the MRT Blue Line was the extension west to Lak Song and from Tao Poon to Tha Phra with 19 stations consisting of four underground and 15 elevated stations. Contracts for civil work were divided into five contracts. Successful bids were announced in late 2010. On 17 February 2011, the five contracts were signed.Bangkok to complete Blue Line loop Railway Gazette International 19 March 2011

ContractNotesLength 1 Hua Lamphong to Sanam Chai (2 stations underground, Bt11.65 billion, contractor ITD) 2 Sanam Chai to Tha Phra (2 stations underground, Bt10.82B, contractor CK) 3 Tao Poon to Tha Phra (8 stations elevated, Bt11.40B, contractor SH-Unique Joint venture) 4 Tha Phra to Lak Song (7 stations elevated, plus a park-and-ride venue and maintenance yard, Bt13.43B, contractor STEC) 5 Rolling stock and systems, Bt5.17 billion, contractor CK. Construction commenced in mid-2011 for a scheduled 2016 opening but this was subsequently delayed to 2019–2020. In August, 2017, BEM awarded the automatic fare collection ticket for the extension to Thales which to install its TransCity system. The contract also includes ongoing training and future support. In September 2017, BEM announced that Siemens and ST Electronics Thailand were awarded the contract (Contract 5) to supply, install and maintain the E&S systems, install station platform screen doors and fit out the depot. Siemens also won the contract to supply 35 sets of rolling stock.

In 2017, the cabinet approved the amendment of the Blue line concession with BEM, and the new agreement was signed on 31 March 2017. Under the new agreement, BEM will operate the new extensions from Hua Lamphong to Lak Song and Bang Sue to Tha Phra. The concession period was extended from 2029 to 2050.

The extension was opened in stages. The Hua Lamphong - Lak Song extension was opened for trial service on 29 July with full commercial services commencing 29 September 2019.

The Tao Poon - Tha Phra extension opened for limited trial services from 29 November 2019 with full commercial services commencing on 30 March 2020.

Phutthamonthon Sai 4 Extension (future)

Since 2011, there were requests from residents to extend the line farther from Lak Song west to the Phutthamonthon area. The MRTA indicated a willingness to undertake this extension. In July 2014, the MRTA submitted a request for 84 million baht to complete a route survey for a , 4 station extension from Lak Song to Phutthamonthon Sai 4. In late March 2017, the Transport Ministry stated that this extension would be submitted to Cabinet for approval in April 2017 with construction expected to commence in the 4th quarter of 2017. However, the tender was subsequently delayed until late 2019 due to other projects. In late 2019, the project was put on hold due to government budget constraints. In August 2021, the MRTA confirmed that the extension was not a priority especially given delays with the tenders for the MRT Orange and MRT Purple line extensions. CodeStation NameEnglishThai Phutthamonthon Sai 2พุทธมณฑลสาย 2 Thawi Watthanaทวีวัฒนา Phutthamonthon Sai 3พุทธมณฑลสาย 3 Phutthamonthon Sai 4พุทธมณฑลสาย 4

Opening timeline