The SRT Dark Red Line starts at the new Krung Thep Aphiwat Central Terminal heading north for 26 km (16.16 mi) via Don Mueang and terminates at Rangsit in Pathum Thani Province. The line will then be extended 8.84 km (5.49 mi) north from Rangsit to the Thammasat University Rangsit campus.

From Krung Thep Aphiwat Central Terminal the line will be extended 11 km (6.84 mi) south via Phaya Thai to Hua Lamphong station. The final section of the line will run southeast from Hua Lamphong via Wongwian Yai for 39 km (24.23 mi) to Maha Chai in Samut Sakhon Province along the current Maha Chai railway alignment.

When fully completed, the line will run on a north–south axis through Bangkok, from Thammasat University's Rangsit campus in Pathum Thani Province to Maha Chai in Samut Sakhon Province for a full length of nearly 87 km (54.06 mi).

In 2004, in conjunction with OTP, the SRT began formulating plans for a new, modern suburban network in Bangkok along existing SRT alignments to replace the existing, limited services. On 7 November 2006, the Thai Cabinet passed a resolution to approve the framework of the new network with the SRT Light Red line being DMU operations while the SRT Dark Red Light would be EMU.^[10] At the time, it was expected that the full line could be completed within 15–20 years. In February 2009, the Thai government secured a 24 billion baht (US$685 million) loan from the Japanese Government for the initial segment of the 87 km (54 mi) line.^[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.^[13] A construction period of 3 years was stated. By the end of 2013, the project was only 3% done & already months behind schedule due to delays in removing the Hopewell Pillars.^[14] 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 Grand Station to cater for future HSR lines.^[15] The 8.14 billion baht requested composed of 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 an extra 473 million baht for rolling stock (Contract 3).

The budget for the 1st Phase of the Dark Red Line project progressively increased due to numerous delays and further redesigns of the project. From an initial estimated 59.89 billion baht in 2007, to 75.55 billion baht in 2009 and to 80.38 billion baht in 2012. An additional requested 8.14 billion baht to increase the number of tracks increased the final budget to 88.52 billion baht.

The 26 km (16 mi), 10 station Bang Sue to Rangsit section finally started construction in May 2013 with a scheduled construction period of just over 3 years to be completed by the end of 2016. 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.^[16] 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.^[14]

In June 2014, the SRT requested an additional 8.14 billion baht to modify the Dark Red line to 4 tracks instead of 3.Aaccordingly, redesign all stations and to provide for the longer platforms for the Bang Sue Terminal to cater for future HSR lines.^[15] 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.

In September 2017, civil works progress was stated to be 88.63%.^[17] By the end of September 2019 civil works were almost fully complete at 99.56% and Electrical and Signalling installation was at 45.60%.^[18] By July 2020, S&E installation was at 85.12% completion.^[19] 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.^[20]

Contract 1 of the project was 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.^[21] 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 the future planned High Speed lines.^[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.^[23][24][25] In July 2014, construction was 10% behind schedule.

The new station was originally scheduled to open by end of 2016 but the above-mentioned redesign work and other delays resulted in an expected 2020 opening date. In September 2017, construction progress of the station civil works was at 57.50%.^[17] By the end of September 2019 civil works were at 86.01%.^[18] By July 2020, civil works were almost complete at 99.8%.^[26]

The 3rd contract for the Dark Red Line was for electrical and systems (E&S) and procuring EMU rolling stock. An overhead catenary electrical system with 25 kV 50 Hz AC was specified. In April 2014, only 2 bidders remained but one of the bidding consortiums was disqualified on due to the fact that one of the consortium members (Maru Beni Corp) had convictions for bribery in an Indonesian bidding process.^[14]

This left MHSC Consortium (Mitsubishi and Hitachi and Sumitomo) as the sole bidder qualified for the contract. However, their bid of 28,899 million Baht was above the SRT median price of 26 billion baht which was set in 2010. 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.^[27] However, the coup of May 2014 delayed finalization leading to further review and negotiations. By mid 2016, negotiations had concluded and Hitachi promised that all rolling stock for the Dark Red line would be delivered by 2020.^[28] The contract specifies 25 EMUs consisting of ten 4 car sets and fifteen 6 car sets for 130 cars in total.

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.^[30] By March 2020, 5 sets had been delivered.^[31]

As of July 2020, 13 sets - 7 of the 6 car sets and 6 of the 4 car sets - of the total 25 sets of rolling stock had been delivered with a further 2 sets due to be delivered by August 2020.^[19] By the end of September 2020, 21 sets had been delivered - 13 of the 6 car sets and 8 of the 4 car sets - with the final 4 sets to be delivered in October.^[32]

Services operate between 5:30am to 12am. Headways are every 20 minutes except for the peak periods (7am to 9am and 5pm to 7pm) where services depart every 12 minutes. The Line has a flat fare of 20 Baht. The SRT also offers a 30 day integrated Transit Pass which can be used for 50 trips and for travel on all BMTA buses.

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.^[33] By the end of September, this had increased to around 4,500-5000 passengers a day.^[34] For the first 10 months of operations to September 2022 both Red lines carried over 3.2 million passengers and services ran on time 99.45% of the time according to the SRT.^[35] By the end of September 2022, the line averaged only 309,000 passengers a month.^[36] By January 2023, the average number daily passengers rose to around 22,000 to 23,000 on weekedays.^[37] At the end of April 2023, daily weekday passengers were 4000 for Don Mueang station and 2500 for Rangsit station.^[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.^[39] This was delayed and then due to be tendered in the 2nd half of 2019.^[40] However, the tender has yet again been further delayed until 2021 as the Transport Minister has requested that the new Department of Railways investigate conducting PPP tenders for this extension.^[41] 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.^[42] 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.^[8] However, this was delayed yet again to an initial October 2022 tender release^[43] and later a December 2022 date.^[44] 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.^[45] 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 is unlikely that the tenders will 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 could be issued in the first quarter of 2024.^[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.^[39] The extension to Thammasat University was then to be tendered in the 2nd half of 2019.^[40] 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.^[41]

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.^[39]

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^[47] 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.^[48]

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.

Currently, services operate as all stops. Express trains will likely enter operation after additional phases are completed.

Code	Station Name	Thai	Express Train (planned)	City Train	Transfer	Location
Phase II - Northern extension
RN14	Thammasat University	มหาวิทยาลัยธรรมศาสตร์ ศูนย์รังสิต	●	●		Pathum Thani
RN13	Chiang Rak	เชียงราก		●
RN12	Bangkok University	มหาวิทยาลัยกรุงเทพ		●
RN11	Khlong Nueng	คลองหนึ่ง		●
Phase I
RN10	Rangsit	รังสิต	●	●		Pathum Thani
RN09	Lak Hok (Rangsit University)	หลักหก (มหาวิทยาลัยรังสิต)		●
RN08	Don Mueang	ดอนเมือง	●	●	Don Mueang International Airport  ARL  (preliminary works)	Bangkok
RN07	Kan Kheha	การเคหะ		●
RN06	Lak Si	หลักสี่		●	MRT
RN05	Thung Song Hong	ทุ่งสองห้อง		●
RN04	Bang Khen	บางเขน		●	MRT : Bang Khen (design phase)
RN03	Wat Samian Nari	วัดเสมียนนารี		●
RN02	Chatuchak	จตุจักร		●
RN01	Bang Sue (Krung Thep Aphiwat)	กรุงเทพอภิวัฒน์	●	●	SRT   MRT : Bang Sue  ARL  (preliminary works)
RS01
Phase III - Southern extension
RS02	Pradiphat	ประดิพัทธ์		●	SRT	Bangkok
RS03	Sam Sen	สามเสน		●	SRT
RS04	Ratchawithi	ราชวิถี		●	SRT
RS05	Yommarat	ยมราช		●	MRT : Yommarat (tender awarded)
RS06	Yot Se	ยศเส		●	BTS : Yot Se (proposed extension)
RS07	Hua Lamphong	หัวลำโพง	●	●	MRT : Hua Lamphong
Phase IV
RS08	Khlong San	คลองสาน		●	BTS	Bangkok
RS09	Wongwian Yai	วงเวียนใหญ่		●	BTS : Wongwian Yai  MRT : Wongwian Yai (under construction)
RS10	Talat Phlu	ตลาดพลู		●	BTS : Talat Phlu
RS11	Taksin	ตากสิน	●	●	BTS : Wutthakat
RS12	Chom Thong	จอมทอง		●
RS13	Wat Sai	วัดไทร		●
RS14	Wat Sing	วัดสิงห์	●	●
RS15	Bang Bon	บางบอน		●
Phase V
RS16	Rang Sakae	รางสะแก		●		Bangkok
RS17	Rang Pho	รางโพธิ์	●	●
RS18	Sam Yaek	สามแยก		●
RS19	Phrom Daen	พรมแดน		●
RS20	Thung Si Thong	ทุ่งสีทอง	●	●		Samut Sakhon
RS21	Bang Nam Chuet	บางน้ำจืด		●
RS22	Khok Khwai	คอกควาย		●
RS23	Ekkachai	เอกชัย	●	●
RS24	Maha Chai	มหาชัย	●	●
Phase VI (Unlikely to be built)
x	Tha Chalom	ท่าฉลอม				Samut Sakhon
x	Nok Lek	นกเล็ก
x	Si Kot	สีคต
x	Bang Krachao	บางกระเจ้า
x	Ban Bo	บ้านบ่อ
x	Bang Thorat	บางโทรัด
x	Ban Ka Long	บ้านกาหลง
x	Ban Na Khwang	บ้านนาขวาง
x	Ban Na Khok	บ้านนาโคก
x	Ban Ket Muang	บ้านเขตเมือง				Samut Songkhram
x	Lad Yai	ลาดใหญ่
x	Bang Krabun	บางกระบูน
x	Mae Klong	แม่กลอง
Phase VII (Unlikely to be built)
x	Bang Khan Thong	บางขันทอง				Samut Songkhram
x	Plai Phong Phang	ปลายโพงพาง
x	Wat Pleng	วัดเพลง				Ratchaburi
x	Pak Tho	ปากท่อ			SRT Southern line