The Yellow line was first proposed in the mid-1990s as a monorail line by Japanese consultants with little or no progress for 10 years. In 2004, the
Office of Transport and Traffic Policy and Planning (OTP) reworked the proposal as a heavy rail line with underground and elevated sections as part of the 10 line metro plan which was taken to the February 2005 election. In 2009, it was suggested to change the line to an elevated monorail line as a cost saving measure. 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 reduce land appropriation costs. In June 2012, the MRTA contracted consultants to undertake detailed designs of the line. In February 2013, OTP stated that the tender for the Yellow Line should be ready by late 2013 for tender in early 2014. By August 2013, this timeline had changed to a mid 2014 tender date. However, similar to the
MRT Pink Line, delays in finalising the technical requirements of the tender in relation to the selection of monorail rolling stock which determines the type of track to be constructed resulted in further delays. The subsequent political turmoil of late 2013 and early 2014 caused even more delays. Thereafter, a coup in May 2014 resulted in a new military administration and the tender being deferred while a review of all mass transit projects was undertaken for a period of 18 months. The MRTA was instructed by the junta government to change to a PPP tender process which was subsequently not released until mid-2016. In early December 2016, The BSR consortium consisting of
BTS Group Holdings (75% majority stake) with Sino-Thai Engineering and Construction (STEC), and Ratchaburi Electricity Generation Holding (RATCH), won the bid to construct and operate the Yellow Line. The BSR also won the bid to build and operate the
MRT Pink Line. On June 16, 2017, the contract was signed for the project between the
Mass Rapid Transit Authority of Thailand and BSR consortium. The BSR established the Eastern Bangkok Monorail Company Limited (EBM) to operate the line. A proposed 2 station extension of the route north from the Lat Phrao terminus to link to the BTS
Sukhumvit Line at Ratchayothin station has also been proposed by the BSR Joint Venture. However, this extension is opposed by the
MRT Blue Line operator
BEM due to concerns regarding loss of revenue. Construction of the Yellow Line began in March 2018. The public trial run started on 3 June 2023, between Hua Mak and Samrong. The Phawana-Si Kritha section opened on 12 June, The extension was approved in February 2021, with construction set to be completed in 2024. However, EBM has repeatedly denied MRTA's request to provide compensation for BEM, which may result in a cancellation of the extension if no decision has been made. Regardless, due to lower-than-expected passenger numbers, MRTA decided to put the extension plan on hold, as the Yellow Line needs at least 100,000 passengers per day to ensure the investment feasibility of the project.
Southern extension A future extension from
Samrong station across the
Chao Phraya River to link with the
MRT Purple Line at Rat Burana was canvassed by the OTP in the early 2010s. However, the current location and design of Samrong station excludes any further extension of the line west of Sukhumvit Road. ==List of stations==