[4] The line runs from Siam Station, where it interchanges with the Silom Line, and continues both northwards along Phaya Thai and Phahon Yothin Roads to Khu Khot in Lam Luk Ka District, Pathum Thani, and eastwards along Rama I, Phloen Chit and Sukhumvit Roads, through Bang Na District to Kheha Station in Samut Prakan.

When it opened, the line ran from Mo Chit to On Nut covering a total distance of 16.7 km (10.4 mi) with 17 stations.

The line rapidly expanded from 2011 until 2021, tripling its distance through three extension projects and reaching a total length of 51.17 km (31.80 mi) with 47 stations.

Additionally, there were protests demanding that the construction of both Skytrain lines be built underground instead of being fully elevated.

Therefore, several alternative locations for the depot were considered, such as a vacant land plot in Soi Rangnam owned by the Crowned Property Bureau or the Phra Khanong pumping station in Sukhumvit 50.

The delay in opening prompted the BMA to offer free travel on this extension until the end of 2011 as compensation.

In April 2013, the MRTA awarded Ch Karnchang the contract for track laying and electrical systems.

[9] In January 2017, it was announced that the opening of the extension could be further delayed to 2018-2019 due to budgetary concerns and ongoing disputes between the MRTA and BMA.

However, due to a combination of changes in government, a prolonged environmental study, and problems with locating a suitable train depot the extension was continually delayed.

A tender was finally released in January 2014[13] with an April deadline before being delayed until late May 2014 due to concerns from bidders.

[20] Testing of the final 9.8 km, seven-station-section from Wat Phra Sri Mahathat (N17) to Khu Khot (N24) began on 5 October.