The Don Mueang–Suvarnabhumi–U-Tapao high-speed railway, officially known as the High-Speed Rail Linking Three Airports Project (Thai: โครงการรถไฟความเร็วสูงเชื่อม 3 สนามบิน) is the second high-speed rail line project in Thailand, being due to open in 2029^[1] between Don Mueang International Airport, Suvarnabhumi Airport and U-Tapao International Airport. It will be operated by Asia Era One Company Limited, a special-purpose vehicles by the consortium of Charoen Pokphand Group Company, Limited (CP) and partners Ch. Karnchang PLC. (CK), Bangkok Expressway and Metro PLC. (BEM), Italian-Thai Development PLC. (ITD) and China Railway Construction Corporation Limited (CRCC).

This project is part of the Eastern Economic Corridor of Thailand.

A HSR line to the eastern seaboard was first proposed in 1996 but there was no progress for over a decade. In 2009, the government requested the Office of Transport and Traffic Policy and Planning (OTP) to create a plan for new HSR network in Thailand that included an eastern HSR line to Rayong. The route was finalised before the 2011 election with the promise to begin construction the next year if the government was re-elected, but they lost the election. After the 2011 election, the new government reviewed all HSR plans and the SRT stated that the line would be tendered in early-2014.^[2] After the May 2014 coup there were further delays while the military government reviewed all HSR lines, initially deferring all projects. In early-2016, the government agreed to proceed with the eastern HSR route and suggested that it could be extended to Don Mueang International Airport beyond the terminus at Bang Sue Intercity Terminal thus providing a link with three airports.^[3] Extending the line would provide a link between Don Mueang Airport, Suvarnabhumi Airport, and U-Tapao International Airport in Ban Chang District.

During 2017, OTP and the Ministry of Transport in consultation with the SRT agreed that by extending the line to terminate at Don Mueang it would effectively include the long delayed extension of the Airport Rail Link (Bangkok) from Makkasan Station to Don Mueang Airport as part of the project. The Eastern Economic Corridor Office (EEC Office) in October 2017 finalised previous OTP plans to build the 10 station Eastern HSR line linking Don Mueang Airport, Bang Sue, Makkasan, Suvarnabhumi Airport, Chonburi, Si Racha, Pattaya, U-Tapao Airport, and Rayong. In early-2018, the section to Rayong was excluded due to environmental and safety concerns and it was decided that the line would terminate at U-Tapao Airport.^[4]

The SRT stated that the first tenders for the Eastern HSR line are expected to be tendered by May 2018 with a four month auction period before the contract is awarded.^[5] The cost of the project was estimated to be over 200 billion baht, of which the Thai Government would fund 123 billion baht and the private sector estimated to contribute 90 billion baht.^[6][7]

A future phase 2 extension could extend as far as Trat however a 2020 feasibility study raised questions about its value.^[13]