The Indigo Line was a proposed rapid transit service of the Massachusetts Bay Transportation Authority (MBTA) that would have incorporated parts of the former Grand Junction Railroad, the Seaport District's Track 61, a spur to Riverside station and segments of other MBTA Commuter Rail lines within the Greater Boston region of Massachusetts.

[1][2] The MBTA planned to procure diesel multiple units (DMUs) for Indigo Line service with full-build completion projected for 2024.

[6] The remaining necessary funds were put into the MassDOT budget in 2005-06 following a lawsuit by the CLF for inadequate completion of Big Dig mitigation measures.

[7] While plans for the station improvement projects moved forward, community advocates would call for the conversion of the Fairmount Line to rapid transit.

[10] The final recommendation called for a commuter rail station at Everett Street with DMU stops added later at the other locations.

[12] Hybrid rail, which emerged during the early 2000s with systems such as New Jersey Transit's River Line, was intended to be a low cost alternative to implementing high-frequency services on existing right-of-ways through the use of FRA non-compliant DMUs.

Initial BRA proposals called for the use of FRA non-compliant rolling stock in order to reduce procurement and operational costs.

Non-compliant DMUs are lighter than FRA compliant vehicles and often have low-floor boarding; however, they are not permitted to operate in mixed traffic with conventional rail equipment.

The lack of additional respondents was identified as a due diligence issue, as it negated the MBTA's ability to negotiate on price and delivery.

Despite these concerns, an order of Nippon Sharyo DMUs for Sonoma–Marin Area Rail Transit in 2014 included an option for 18 cars to be provided to the MBTA for the Indigo Line service.

[17][16] Development on the Indigo Line proposal immediately stalled following its inclusion as a capital project within MassDOT's five-year plan.

[18] The abandonment of the project was attributed to high costs of acquiring specialized rolling stock along with a lack of demand for the routes it would serve.

[19] The accusation came in response to a joint Department of Justice and FTA investigation of MBTA Commuter Rail operator Keolis into whether train cancelations on the Fairmount Line were in violation of Title VI in 2016.

[24] As originally proposed, the Indigo Line is no longer in active development; however, many attributes of the project are succeeded within MBTA regional rail planning.

[26] On March 16, 2024, MBTA Commuter Rail operator Keolis solicited a proposal that outlined the implementation of 20-minute bidirectional BEMU services on the Fairmount Line by FY 2028 as leverage for contract renewal.

In July 2024, the MBTA board approved the $54 million Fairmount Line proposal, with new BEMU equipment to enter service in early 2028.

[28][29] The 2023 legislation diverges from a previous MBTA proposal from 2022 that suggested the procurement of battery multiple units for the Fairmount Line as opposed to full electrification.