was first planned by the Thai parliament in 2010 with a proposal of five routes radiating from Bangkok. 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. The first 86 km section from Bang Sue Grand Station to Ayutthaya was planned to be tendered in late-2013; however following the 2014 Thai coup d'état, plans were deferred. In November 2014, Thailand and China signed a memorandum of understanding agreeing to construct the Thai portion of the transnational railway running from Kunming, China to the Gulf of Thailand. In November 2015, both parties agreed to a division of labour. Under the framework, a joint venture will be set up to run the project. China will conduct feasibility studies, design the system, construct tunnels and bridges, and lay track. Thailand will conduct social and environmental impact studies, expropriate land for construction, handle general civil engineering and power supply, and supply construction materials. China will operate and maintain the system for the first three years of operation. Both countries will share responsibility from the third to seventh years. Afterwards, Thailand will have sole responsibility, and China will advise. China will provide training for operations and maintenance. Dual standard-gauge tracks will be laid in the project. In Thailand, two routes will diverge at a junction in Kaeng Khoi District in Saraburi Province. One will connect Bangkok to Kaeng Khoi. The other route will connect Kaeng Khoi with Map Ta Phut of Rayong Province. From Kaeng Khoi tracks will lead north to Nakhon Ratchasima and on to Nong Khai Province. The Bangkok to Nong Khai HSR line will connect to the Vientiane–Boten railway from Vientiane to the northern Lao border and the line from the Lao border to Kunming.
Construction divided into four sections: Bangkok-Kaeng Khoi, Map Ta Phut-Kaeng Khoi, Kaeng Khoi-Nakhon Ratchasima, Nakhon Ratchasima-Nong Khai. Construction of Thailand's portion of the railway system started in December 2017 and is expected to take three years for phase 1 to Nakhon Ratchasima. The first contract consisted of a 3.5 km section from Klong Dan to Pang Asok with a budget of 371 million baht after a nine percent decrease from the initial budget. In February 2018, the Thai transport minister stated that the contract for the first phase, the 253 km Bangkok to Nakhon Ratchasima section, will be issued by May 2018. The second contract will cover the 11 km section from Sikhio to Kut Chik. The first phase of the line to Nakhon Ratchasima has been divided into 14 contracts, with contracts 3-7 to be auctioned by June 2018, while contracts 8-14 will be auctioned before the end of 2018.
