As of 2018[update], the only station in service on the Blue Line which is not fully accessible is the downtown Boston terminus Bowdoin.

Court Street had pedestrian access to Scollay Square station (now Government Center) but transfers to the East Boston Tunnel required an additional fare of 1 cent.

[11] The East Boston Tunnel was originally planned to be operated with high-floor metro rolling stock and connected to the then-planned Cambridge Elevated line.

When that plan was dropped in 1903 due to a disagreement between the Boston Transit Commission and the BERy, the stations were built with low platforms.

[12]: 19  Large bi-loading streetcars (with high floors but capable of loading from low platforms), which incorporated many attributes from metro cars used on the Main Line Elevated, began use in 1905.

[12]: 118–119 [13]: 14  However, neither these nor the large center-entrance cars introduced in 1917 (which were designed for multiple unit operation) could fully handle the crowds.

[18] From 1952 to 1954, a surface-level extension was constructed along the recently-defunct Boston, Revere Beach and Lynn Railroad, from Maverick to the current terminus at Wonderland.

[6]: 51  The first above-ground station on the new extension served Boston's Logan Airport, and was the first American urban transit connection to a commercial airfield.

[6]: 51  Beyond Maverick, the power feed was changed from third rail to overhead catenary (both at 600 V DC) to reduce the risk of winter ice buildup due to proximity to the ocean.

[6]: 51 The line was officially renamed the East Boston Tunnel & Revere Extension by the MTA in 1952, and designated as "Route 3" on system maps.

The color blue represented water, as the line passes under Boston Harbor and travels near the coast for much of its length.

By the 1990 passage of the Americans with Disabilities Act, only Suffolk Downs and the eastbound platform at State were accessible, with Wonderland then under construction.

[21][22] In 1988, the MBTA began planning for accessibility modifications as well as platform extensions to accommodate 6-car trains - similar to what had been done on the Orange and Red lines.

[23] In 1989, the MBTA awarded design contracts for modernization and platform lengthening (to accommodate six-car trains) at nine Blue Line stations.

[4] The first main construction phase of the project began on June 25, 1994, the line was cut back to Orient Heights at all times to permit reconstruction of the outer stations.

Beachmont and Wood Island were completely rebuilt, while Suffolk Downs, Revere Beach and Wonderland were renovated.

[24] A simultaneous $8 million noise reduction program added sound walls at Beachmont and Orient Heights, rubber mats under tracks, and soundproofing of nearby homes.

[30][31] Sequential closures of Wonderland, Wood Island, Revere Beach, and Beachmont stations took place in June through November 2008 for platform repairs.

[36] This left all Blue Line stations accessible except for Bowdoin, which also has an eastbound platform that cannot be modified for six-car trains.

[39][41] An additional closure of the outer portion of the line from May 22 to June 8, 2022, allowed for repairs to the Suffolk Downs footbridge.

The land to extend the line was purchased for the initial construction of the Revere Extension, but due to budgetary constraints Wonderland station was designated the northern terminus.

An alternative route would extend the line alongside Revere Beach Boulevard through Point of Pines and the Lynnway, along the remainder of the BRB&L right of way.

[50] A 2004 state bond bill authorized $246.5 million on the condition of finding 50% non-state matching funds (which presumably would come from the federal government).

[46] The Draft Environmental Impact Report was expected to be complete by the end of 2008,[50] but was delayed as planners focused on meeting the legal deadline for the Green Line extension to Somerville and Medford.

[59][60][61] An Expanded Environmental Notification Form was released in September 2007[62]: 1.1 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.

The new Blue Line level at Charles/MGH would have entrances from the existing lobby and a new headhouse in an MGH development on the north side of Charles Street.

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.

[6]: 32  When the Blue Line eventually got its own maintenance shops, the connection was removed and the ramp portal was permanently covered in 1952.

Uniquely among MBTA rolling stock, Blue Line cars use both third rail power and pantograph current pickup from overhead catenary wires.

[18] Parts of scrapped cars are used to maintain Orange Line 1200 series rolling stock, which were built at the same time by Hawker Siddeley and used many of the same components.