[7] A much delayed 8.84 km (5.49 mi) extension of the line from Rangsit to Thammasat University was due to be tendered in June 2022.

From Krung Thep Aphiwat Central Terminal the line will be extended 11 km (6.84 mi) south via Phaya Thai to Hua Lamphong station.

[11][12] The first Phase from Bang Sue to Rangsit was approved in 2010 but delayed due to a complicated 2.5 year contractual dispute.

The 26 km (16 mi), 10 station Rangsit to Bang Sue section finally started construction in May 2013 after new contracts were signed in January 2013.

[22] (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 the Thai army Explosive Ordnance Teams.

[14] This left MHSC Consortium (Mitsubishi and Hitachi and Sumitomo) as the sole bidder qualified for the contract.

The MHSC Consortium argued that their bid reflects 2013 prices after the minimum wage increase of January 2012.

Finally in July 2014, after a prolonged 2-year delay in the bidding process, JICA approved the loan for Contract 3.

By mid 2016, negotiations had concluded and Hitachi promised that all rolling stock for the Dark Red line would be delivered by 2020.

In late September 2019, the first 2 sets of rolling stock were shipped from Japan[29] and both arrived in Thailand at Laem Chabang port on 12 October 2019 for shipment to Bangkok.

The SRT also offers a 30 day integrated Transit Pass which can be used for 50 trips and for travel on all BMTA buses.

[38] In July 2016, the Thai Cabinet approved the first section of the southern extension from Bang Sue to Hua Lamphong.

However, the 10 km (6.2 mi) 4 station northern extension from Rangsit to Thammasat University will be built first and was originally expected to be tendered by September 2018.

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.

[45] At the same time, the SRT decided to defer the bidding for the 50 year operation concession and new EMUs to December 2024.

With the 14 May 2023 national elections and the expected time frame in the formation of a new government, it is unlikely that the tenders will be issued until late 2023.

[9] Construction segments based on M-Map:[46] The 8.84 km (5.49 mi), 4 station extension was approved by Cabinet in 2016 with an expected tender by September 2018.

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.

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.