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

Track 61 is an industrial rail terminal track in South Boston, Massachusetts, also known as the Boston Terminal Running Track. Track 61 is the last remnant of the vast rail yards that once covered much of the South Boston waterfront. Track 61 legally begins at Summer Street, while the line from Bay Junction to Summer Street is the Boston Terminal Running Track and Terminal Yard. However, the names are frequently used interchangeably.