The line is 23 km (14 mi) long,[2] serving the north-western area of Bangkok running from Tao Poon to Khlong Bang Phai in Nonthaburi Province.

The cost of the southern extension is 82 billion baht and it is planned to open at the end of 2027.

It continues along Rattanathibet Road, passing through a large residential area in Bang Yai District and crosses the Chao Phraya River on a bridge parallel to the Phra Nang Klao Bridge and runs towards Nonthaburi Civic Center station.

On 31 March 2017, the fifth contract was effectively terminated and became part of a new Blue Line concession agreement.

In June 2013, five people were injured when six beams fell onto vehicles below Tao Poon station.

[23] When the Blue line extension finally opened a year later, on 11 August 2017,[24][25] ridership significantly increased.

[44] Contactors were issued with a Notice to Proceed on 25 April 2022 giving them access to sites along the route with actual construction planned to begin from August 2022.

[50] Formerly Si Phon Sawan Formerly Phan Fa Formerly Samre Formerly Rama 9 Bridge [2] In November 2013, BEM contracted with Marubeni Corp, Toshiba Corp, and East Japan Railway Company to install E&S systems, supply 21 three-car trains (total of 63 cars) rolling stock, and provide maintenance for 10 years as part of a contract worth 12.6 billion baht.

[53] East Japan Railway train manufacturing subsidiary J-TREC built them in Yokohama with the first sets delivered in early 2016.

[10] Trains are powered by 750 V DC via third rail system, are air-conditioned and capable of traveling at up to 80 km/h (50 mph).

[56] After the opening of the Blue Line extension to Tao Poon 1 year later in August 2017, daily average ridership increased significantly from 33,000 to 50,000.

[57] In August 2018, the MRTA Deputy Governor stated that daily ridership had increased to 60,000 each weekday.

This measure aims to address the worsening air pollution caused by surging dust levels across all districts of the capital.