The first line of the network is under construction from Bangkok to Nakhon Ratchasima, with a planned maximum operational speed of 250 km/h (155 mph).

[1] In March 2013, the transport minister revealed that only one company would be selected to run all high-speed train routes, scheduled to be operational between 2018 and 2019.

However, a seven-month political crisis involving the dissolution of parliament and an annulled February 2014 election culminated in a military coup in May 2014.

Following the military coup of May 2014 and his elevation to the office of prime minister, Gen Prayut Chan-o-cha proposed connecting Bangkok to two popular resort cities, Pattaya and Hua Hin, by high-speed rail.

They assumed that, for the Bangkok-Pattaya line, trains would run through Chachoengsao, Chonburi, and Pattaya, terminating in Rayong, a distance of 194 km (121 mi).

The route to Hua Hin would be 209 km (130 mi) in length with an investment cost of about 98 billion baht and an EIRR of 8.1%.

The office concluded that these routes would be of little interest to private investors due to the high investment required coupled with a low rate of return.

China would conduct feasibility studies, design the system, construct tunnels and bridges, and lay track.

During 2017, OTP and the Ministry of Transport in consultation with the SRT agreed that extending the line to terminate at Don Mueang 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.

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.

[27] The Charoen Pokphand (CP) Group-led consortium consisting of Italian-Thai Development, China Railway Construction Corporation Ltd, CH.

"[28] The project was eventually approved in October 2019 as a public private partnership between the Thai government and Charoen Pokphand/China Railway Construction Corporation.

[32] Japan would provide Shinkansen technology for a high-speed rail link between Bangkok and the northern city of Chiang Mai.

Seven stations are planned for this segment: Krung Thep Aphiwat, Don Mueang, Ayutthaya, Lopburi, Nakhon Sawan, Phichit, and Phitsanulok.

[33] The Thai government announced in September 2019 that it may cancel Bangkok-Chiang Mai high-speed rail project after private investors declined to invest.

[37] On 14 December 2022, the Department of Railways is discussing with MLIT-JICA to speed up the conclusion of economic and financial feasibility studies for the high-speed rail between Bangkok and Chiang Mai, to be completed in March 2023.