The project is suggested to relieve pressure on Downtown Boston stations, overcrowding on the Green Line (which is currently the most convenient way to transfer between the two lines), and to connect communities unlinked by public transit, including Cambridge and East Boston.

The lack of progress on the connector has been controversial, as the project has been repeatedly delayed or put on hold despite a significant amount of support from politicians and the public.

[8] It currently takes an average of 11 minutes to travel between Government Center and Charles/MGH during rush hour, by way of the Green and Red lines.

This connection was never used in passenger service, but was used to transport Blue Line cars to the Eliot Street Yard maintenance shops then located near Harvard Square station.

[10]: 32  When the Blue Line eventually got its own maintenance shops, the connection was removed and the ramp portal was permanently covered in 1952.

An underground Blue Line platform at Charles/MGH is planned to be built underneath Cambridge Street to the east of the main headhouse.

This will provide a direct transfer between the Blue and Red lines without passengers having to exit fare control.

By the mid-1970s, the rise of Kendall Square as a major employment center, the coming of the Red Line extension to Alewife, and increased traffic to and from Logan Airport created enough demand to justify a direct connection between the Red and Blue lines.

The reconstruction of Charles/MGH station (rebuilt 2003–2007) was designed to accommodate a future Blue Line platform to the east of the existing headhouse.

[20][21] The 2010 Draft Environmental Impact Report (DEIR) instead called for a pair of deeper tunnels bored by a tunnel boring machine (TBM), starting east of Bowdoin station and passing underneath the existing platform.

[27][28] A conceptual design completed in 2020 called for cut-and-cover construction, with Bowdoin station still planned for closure.

The new Blue Line platform at Charles/MGH would have egresses from the existing lobby and an under-construction MGH development on the north side of Charles Street.