SRT Dark Red Line

The 26 km (16 mi), 10 station Bang Sue to Rangsit section finally started construction in May 2013 with a scheduled construction period of 3 years. Construction works were delayed by 2 months due to site access issues for the contractor and delays related to removing slum dwellers residing within the right of way. However, it was hoped that the use of some of the old Hopewell pillars would speed up the initial construction timetable.[17] By the end of 2013, the project was only 3% done & already months behind schedule due to a longer timetable in removing the Hopewell Pillars.[18]

In June 2014, the SRT requested an additional 8.14 billion baht to modify the Dark Red line to 4 tracks instead of 3, to accordingly redesign all stations and to provide for the longer platforms for the Bang Sue Terminal to cater for future HSR lines.[19] 8.140 billion baht request is; 4.32 billion baht for Contract 1 (modifications to Bang Sue Grand Station to cater for High Speed trains); 3.35 billion Baht for Contract 2 (4th track and stations redesign) and 473 million baht for Contract 3.

As of September 2017, civil works progress was stated to be 88.63%.[20] By the end of September 2019 civil works were almost fully complete at 99.56% and Electrical and Signalling installation was at 45.60%.[21] By July 2020, S&E installation was at 85.12% completion.[22] By October 2020, 90% of the power supply for the line had been installed by the Metropolitan Electricity Authority (MEA) with all installation due for completion by November 2020.[23]

Contract 1 of the project is for the construction of a new, 4 level Intercity Terminal to cater for all SRT Intercity Trains, SRT Red Line suburban trains and the yet to be built Airport Rail Link (Bangkok) extension. The delayed contract process was finally signed in January 2013.[24] In August 2013, the SRT sought extra funds for the project to be redesigned in order to extend upper level platforms to a 400–600 m length in order to accommodate future planned High Speed Lines.[25] (Funding was finally requested in June 2014 - see above section)

In March 2013, the new Bang Sue Grand Station started construction. Between March and June, excavation works for the foundation of the Terminal were delayed by the unearthing of numerous World War II unexploded bombs which required safe removal by army Explosive Ordnance Teams.[26][27][28] As of July 2014, construction was 10% behind schedule.

The new Bang Sue Grand Station was originally scheduled to open by end of 2016 but the above-mentioned redesign work and other delays have resulted in an expected 2020 opening date. As of September 2017, construction progress of the station civil works was at 57.50%.[29] By the end of September 2019 civil works were at 86.01%.[30] As of July 2020, civil works were almost complete at 99.8%.[31]

On the first full day of free trial operations on 3 August 2021 total passengers numbered 2914, on 4 August 2856 passengers used the line.[40] By the end of September, this had increased to around 4,500-5000 passengers a day.[41]

The 8.84 km (5.49 mi), 4 stations was approved by Cabinet in 2016 and was expected to be tendered by September 2018.[48] The extension to Thammasat University was then to be tendered in the 2nd half of 2019.[49] However, the tender has further been delayed until 2021 as the transport minister has requested the new Department of Railways to investigate conducting PPP tenders for this extension.[50]

The 11.0 km (6.84 mi), 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.[51]

The last section of the line will run 39.0 km (24.23 mi) southeast from Hua Lumphong to Maha Chai in Samut Sakhon Province via Wong Wian Yai along the current Maha Chai railway alignment.

The Office of Transport and Traffic Policy and Planning (OTP) have studied the improvement and construction of the Mae Klong railway line[52] 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 31 km (19 mi). 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.[53]

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.