In July 2016, the Thai Cabinet approved the first section of the southern extension from
Bang Sue to Hua Lamphong. However, the 4 station northern extension from Rangsit to Thammasat University (Phase II) will be built first. Construction segments based on
M-Map:
Phase II Rangsit–Thammasat University The , 4 station extension was approved by Cabinet in 2016 with an expected tender by September 2018. The extension to Thammasat University was then to be tendered in the 2nd half of 2019. However, the tender was further been delayed until 2021 as the transport minister requested that the new Department of Railways investigate conducting PPP tenders for the extension. The decision to change the extensions to PPP projects and tender out the operation of the line with a 50 year concession was opposed by the SRT and SRT union. On 10 February 2021, the Department of Railways announced that in April 2021 the SRT would issue the tenders for the north extension to Thammasat University and the south extension to
Hua Lamphong station. However, the PPP tender process was subject to further review. In October 2021, the SRT announced that the PPP tenders would not be released until June 2022 with the aim to sign contracts for the extensions (with 50 year leases) in July 2023. However, this was delayed yet again to an initial October 2022 tender release and later a December 2022 date. In late October 2022, the SRT again delayed the tender time frame to February 2023 with an aim to sign contracts by May 2023 but the time frame was contingent on new Cabinet approval of an updated budget for the extension. At the same time, the SRT decided to defer the bidding for the 50 year operation concession and new EMUs to December 2024. Once the concession is contracted the SRTET will cease to operate the line. With the 14 May 2023 national elections and the expected time frame in the formation of a new government, it became unlikely that the tenders would be issued until late 2023. In late June, a MOT source expressed that the extension is likely be taken to Cabinet in October 2023 for approval of a new budget of 6.5 billion baht to build the extension. If approved, a tender was expected to be issued in the first quarter of 2024. Tenders for Phase II were finally opened in December 2025, with submission due by 29 December 2025 and bidding completed by February 2026.
Phase III Bang Sue–Hua Lamphong The , 7-station southern extension to Hua Lumphong station was approved by Cabinet in 2016. However, the northern extension to Thammasat University will be built before this section.
Phase IV & V Hua Lamphong–Bang Bon–Maha Chai The last section of the line will run southeast from Hua Lumphong to
Maha Chai in
Samut Sakhon Province via Wong Wian Yai along the current Maha Chai railway alignment.
Potential extension Maha Chai–Ratchaburi The Office of Transport and Traffic Policy and Planning (OTP) have studied the improvement and construction of the Mae Klong railway line which will be constructed through Samut Sakhon, Samut Songkhram and Ratchaburi provinces. It will use a bypass route in Samut Sakhon Province by deviating from the original train line for about . Between
Ban Khom railway station and
Khlong Chak railway station, the line will be elevated across Ekachai road and deviate along the route of
Rama II Road around the km 26 + 800 to the 32 + 160 km, then divert to the left to go straight to connect with the original train line. From
Samut Songkhram the suggested route will be constructed across the Mae Klong River. A third option is expected to be in use, which is to bypass Samut Songkhram city by diverging from the original train line about 66 km after passing
Bang Kraboon railway station, which will be an elevated railway along the National Highway No. 325 to cross the Mae Klong Canal and Highway 325 at the intersection to Damnoen Saduak District, approximately 40 + 850 km and crossing the Mae Klong River. It will then revert to ground level and end at
Pak Tho railway station, Ratchaburi which will build a train parallel with the royal highway number 3093 and will have 3 more new railway stations in this section. Expected total value of this project is approximately 42,243 million baht. When completed, it will be a new southern railway line, which will help shorten the original train route, which originally runs through Nakhon Pathom and Ratchaburi before going to Pak Tho Station. The new Southern Railway will reduce the distance by about 43 kilometers and the aim of the project also includes the development of the Southwest Transport Center. However, as of 2016 this planned southern extension is very unlikely to be built as when Cabinet approved the northern extension to Thammasat University and the Southern extension to Hua Lumphong, only the original project scope to Maha Chai was referenced. == List of stations ==