The MRT Blue line is officially known in Thai as rotfaifa mahanakhon (รถไฟฟ้ามหานคร) or "metropolitan electric train", but it is more commonly called rotfai taidin (รถไฟใต้ดิน), literally, "underground train" as it was distinguished from the BTS Skytrain by being completely underground when its first section between Hua Lamphong to Bang Sue opened.

As MRTA's concessionaire, BEM provides M&E equipment, including electrical trains, signalling systems, SCADA, communication, PSD, etc.

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 Blue Line was 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.

Consequently, the entrances to MRT stations in the city are constructed approximately one meter above ground level and are equipped with integrated floodgates to prevent water from entering the system.

Within the stations, maps are displayed to guide passengers, showing local area details and exit locations.

This agreement aims to supply the mass transit system with 452 million kWh of electricity, primarily sourced from solar energy.

Multi-storey park & ride facilities are provided at Lat Phrao and Thailand Cultural Centre.

The first section of the Blue Line extension from Hua Lamphong via Tha Phra to Lak Song opened for full operation on 29 September 2019.

(km) (January 2025) The MRT Blue Line[4] presently runs from Tha Phra to Lak Song and has a carrying capacity of 40,000 people in each direction per hour.

This extension will serve the new Thai parliament construction site at Kiak Kai intersection and will run along Samsen Road, then passing through the historic old quarter of Bangkok Rattanakosin Island and many historical places in Dusit and Phra Nakhon District like the national library, Thewet market, Bang Lamphu, the Democracy Monument, the Golden Mount, Phahurat near Chinatown.

It then runs near the Memorial Bridge over the river to Wongwian Yai roundabout before continuing south to Chom Thong and ends in Rat Burana District.

In December 2011, the MRTA was instructed by the MOT to divide the Yellow Line into two Phases for tender and construction purposes and to control land appropriation costs.

[14] On 16 June 2017, the Pink Line project contracts were signed with the BSR consortium by the Mass Rapid Transit Authority of Thailand with a scheduled operational date of 2022.

The BSR has proposed a 2.8 km spur monorail route to link to Impact, Muang Thong Thani from Si Rat station, which is under construction government.

It then will go into an underground structure along Sam Sen Road, passing the new Thai parliament site, and transfers to the proposed Red inner-city shortcut commuter at Sanghi intersection.

It passes Golden Mount before possible transfers to the extension of the Blue Line at Wang Burapha near Yaowarat Road in Chinatown and the Phahurat textile market.

It continues south to Chom Thong, then on an elevated structure for 5.5 kilometres (3.4 mi) and ends in Rat Burana District, a short distance to the nearby Phra Pradaeng suburban town in Samut Prakan Province.

The Thai Cabinet was expected to approve the Purple Line Southern extension in mid 2017 with a tender due by the 2nd half of 2017.

Then it goes to Thailand Cultural Centre Station, the alignment crosses over and transfers to the current MRT Blue Line.

[25] The Phase 2 Western section extension from Thailand Cultural Centre to Bang Khun Non was released for tender on 3 July 2020 as a Public-Private Partnership project.

It is proposed to run from Khae Rai, Nonthaburi to Bueng Kum with 20 stations and be a monorail line.

[27] However, the Pheu Thai candidate did not win the Bangkok Governor election and the Gold Line was dropped with the Deputy Minister of Transport stating that land appropriation costs were too high.

[31] In early June 2019, the MRTA Board approved the investment plan for the Brown Line and integrated design with the N2 Expressway conducted by EXAT.

140 people were hurt, most of whom sustained only minor injuries and the entire Metro network was shut down for two weeks.

After initial investigations, it was found that the empty train had run into problems shortly before the accident, grinding to a halt on a curve leading to the depot.

The Metro resumed full operation on 1 February 2005, and passenger numbers soon rose back to pre-crash levels, partly due to a temporary promotional fare scheme that allowed passengers to travel any distance on the MRT for only ten baht (~US$0.33).

In September 2017, BEM stated that average daily ridership of the Blue Line had increased from 340,000 to 360,000 after the opening.

[33] In August 2018, the MRTA Deputy Governor stated that daily ridership for the Purple Line had increased to 60,000 each weekday.