Except for a short portion in Milton, it lies entirely within Boston, running southwest from South Station through the neighborhoods of Dorchester, Mattapan and Hyde Park.

Due to neighborhood opposition over its design and location, another planned station, Blue Hill Avenue, did not open until February 25, 2019.

[3] While the line is still among the least-used on the MBTA Commuter Rail system, it has seen significant recent growth from 789 daily riders in 2012 to 2,652 in 2018 and 2,843 in 2022.

The new line, built in 1855, split from the old one at Islington and ran northeast, crossing the Boston and Providence Railroad at Readville.

Three of the former stations – Fairmount, Morton Street and Uphams Corner – were rebuilt with bare asphalt platforms and opened for local commuter service.

[7] Uphams Corner and Morton Street stations closed on January 30, 1981, as part of system-wide cuts that also included the closure of the Woburn Branch.

[5] The route – sometimes called the Dorchester Branch by the MBTA – is used by some rush-hour Franklin/Foxboro Line trains to reduce load on the three-track Southwest Corridor and supplement the shuttle service.

[10][11] During disruptions on the Northeast Corridor north of Readville, Franklin and Providence service is sometimes diverted over the Fairmount Line.

[14] After a quarter-century of debate, plans to replace it with a road bridge at Cedar Place were cancelled in 1941 due to a steel shortage.

[18][19] Although well-trafficked, the bridge deteriorated due to lack of maintenance and became unsafe; lighting was not replaced after persistent vandalism, and robberies were common.

[22] The Commonwealth of Massachusetts agreed in 2005 to make improvements on the Fairmount Line part of its legally binding commitment to mitigate increased air pollution from the Big Dig.

To comply with the State Implementation Plan filed with the federal Environmental Protection Agency, the improvements were required to have been completed by December 31, 2011.

However, in its initial proposal the MBTA did not plan to increase service frequency to match rapid transit lines, nor to install pre-pay fare systems.

The Indigo Line plan also called for the use of diesel multiple unit (DMU) cars for faster acceleration and deceleration, which was considered by the MBTA but not implemented initially because of lack of funding.

[40] The line was shut down on weekends in November and December 2017 for the installation of Positive Train Control equipment in order to meet a 2020 federal deadline.

[45] In January 2020, the MBTA Fiscal Management and Control Board voted to add four weekday Fairmount Line round trips as a one-year pilot program, originally planned to begin on May 18, 2020, but later postponed due to the COVID-19 pandemic.

[46] CharlieCard readers were placed at stations to provide a proof-of-payment ticket that allows a transfer to subway or bus.

[47][5] One additional round trip was added on November 2, 2020, with 45-minute headways all day, as part of a systemwide transition to a regional rail model.

[48] Weekday service was temporarily reduced to hourly headways from December 14, 2020, to April 5, 2021, again as part of systemwide reductions.

[53] Limited Foxboro service also resumed at that time, albeit only with trains running via Ruggles rather than via the Fairmount Line.

As of November 2024[update], Keolis expects to issue a request for proposals for rolling stock in December 2024 and for financing in early 2025.