The BMA Grey Line is a planned monorail line in the north of Bangkok to be built from Watcharaphon to Thong Lo. The route would provide a vital north - south link for a distance of 16.25 km with 15 planned stations and is expected to cost 27 billion baht.^[1] The line would support the growth of residential areas around Pradit Monutham Rd and Kaset-Namawin rd corridors and is forecast to eventually have 370,000 passengers per day. The BMA Grey with interchange with other lines including the MRT Pink Line (under construction), the proposed MRT Brown Line, the MRT Yellow Line (under construction), MRT Orange Line (under construction) and BTS Sukhumvit Line.

The BMA contracted a consultants study into the project which was due to be completed by March 2022.^[2] A new EIA for the line was completed and approved by the National Environment Board in early March 2022.^[3] On 17 March 2022, the BMA held a market sounding seminer with companies interested in potentially bidding for the line and updated that the feasibility study would be completed by early June 2022.^[4]

The BMA proposes to gain Cabinet approval for the line in 2023 and identify private investor partners in 2024-2025 with a view to commencing construction in 2026 in order for the line to open in 2030.^[5] However, property developers have requested that this timeframe be decreased as they believe it is too slow.^[6]

The route runs from Ram Indra Road, in the northeast of Bangkok, where it interchanges with the MRT Pink Line (under construction) running south on Pradit Manuthum Rd along the Chalongrat Expressway corridor. It then interchanges with the proposed MRT Brown Line at Chalongrat Intersection (Kaset-Namawin rd) and continues further south to interchange with MRT Yellow Line at Lat Phrao rd before continuing further south along Pradit Manuthum Rd to interchange with the MRT Orange Line at Rama 9. Finally, the line continues south then south west into Sukhumvit soi 55 before terminating at the downtown district of Thong Lo where it interchanges with BTS Sukhumvit Line.

In August 2011, the newly elected Yingluck Shinawatra government decided to defer plans for the Grey Line and the Light Blue Line, citing that the lines are not a priority within their current 10 lines Master plan. The government suggested the Bangkok Metropolitan Administration pursue construction but stated that the BMA would have to fully fund the line itself.^[7]

The BMA stated in mid 2013 that it considered the line a priority and wanted to progress planning by completing an EIA by the end of 2013 with the aim for the line to be tendered by early 2014.^[8] Construction would have then taken place from 2015 - 2017 for a potential opening in 2018.^[9] In March 2014, the OTP stated that they would contract a consultant to undertake further study of the line.^[10] In May 2014, a consultant was contracted to undertake a financial feasibility study of the line, scheduled to complete the work by the end of 2014.^[11] A second public hearing on the proposed line was held in January 2015.^[12] An environmental impact assessment was completed in September 2016.^[13] However, the EIA did not receive approval from the National Environment Board thus suspending the project.

Also under consideration is a Phase 2 and Phase 3 extension along the length of Rama 3 rd, over the Chao Phraya river and along Ratchadapisek rd (Inner Ring rd) to Tha Phra to replace the BRT along this route. This would enable the Grey Line to interchange with the BTS Silom Line at Talat Phlu station, the future MRT Purple Line (Phase 2) southern extension and link with the MRT Blue Line extension at Tha Phra station. If the Phase 3 extension is built, the total route distance would be 39.9 km. In September 2019, it was announced that the monorail would be built as two disconnected sections and could be ready by 2023.^[21]

A possible Phase 2 extension would extend from Sukhumvit Rd., along Rama IV Rd., and Ratchadaphisek - Rama 3 Rd and end at The Rama IX Bridge at the Chao Praya river and service the Sathupradit area in Yan Nawa District. The Phase 2 extension would result in a route that is approximately 26 km long. It would also essentially replace much of the Bangkok BRT route.

A planned Phase 3 extension from The Rama IX Bridge /Rama 3 rd and would continue along Ratchadapisek rd to link with the BTS Silom Line at Talat Phlu station, the future MRT Purple Line (Phase 2) southern extension and eventually terminate with the MRT Blue Line extension at Tha Phra station. A Phase 3 expansion would result in a total distance for the Grey Line of 39.9 km.

The route has a total of 15 planned stations. A number of station working names were changed by the Department of Rail Transport in August 2022 to better reflect localities of the stations.^[22]

For the potential Phase 2 & 3 sections, 24 stations are proposed. (Note: The Bangkok BRT would be replaced along this section) .