The main $1.08 billion design-build contract was issued in November 2017, with several optional items like platform canopies and a larger vehicle maintenance facility included.

The MBTA expected AFC 2.0 (the Charliecard replacement system) to be fully in place by the time the GLX opened, with passengers able to board and tap their farecards at any door on trains.

Passengers on the Harvard Square and Clarendon Hill surface lines transferred to two-and-three-car subway trains, bringing rapid transit-style service to Lechmere.

In 1945, a preliminary report from the Coolidge Commission recommended nine suburban rapid transit extensions – most similar to the 1926 plan – along existing railroad lines.

[41] The 1962 North Terminal Area Study, claiming that the 1959 Highland branch conversion showed that PCC streetcars were inadequate for suburban rapid transit service, recommended that the elevated Lechmere–North Station segment be abandoned.

[21] The 1966 Program for Mass Transportation, the MBTA's first long-range plan, listed an approximately 1-mile (1.6 km) extension from Lechmere to Washington Street as an immediate priority.

A second phase of the project would extend Green Line service from Washington Street to Mystic Valley Parkway (Route 16) or West Medford.

[43][22] The 1972 final report of the Boston Transportation Planning Review listed a Green Line extension from Lechmere to Ball Square as a lower priority.

A new elevated station would be located on the east side of the O'Brien Highway, with a new vehicle storage yard and maintenance facility to the north.

Both modes were rated medium priority; the Blue Line version was twice as expensive due to the need to tunnel under the Charles River.

[49] In March 2005, with the extension not in the MBTA's five-year plan, the city of Somerville and the CLF filed a lawsuit against the state for breaching the 1991 agreement.

[60][62] The project was widely supported in Somerville, where it was seen as a development catalyst and a correction for past transportation injustices, but was less popular in more suburban Medford.

The EENF identified a Green Line extension with Medford and Union Square branches as the preferred alternative, and sought permission to conduction a single environmental impact report.

The selected interim measures were increased midday frequency in the corridor (Green Line service to Lechmere plus bus routes 80, 88, 91, 94, and 96), the purchase of hybrid vehicles for The Ride, and completion of new parking garages at Salem and Beverly stations.

[80]: 14 The City of Somerville, MassDOT, and the MBTA reached an agreement in August 2012 to open the Union Square Branch by early 2017, with construction to begin in 2014.

[87] In May, MassDOT approved a $118 million contract with CAF USA to build 24 Type 9 LRVs to expand the Green Line fleet for GLX service.

The southern section of the Community Path was removed – prompting criticism from trail advocates – and the number of street access points was reduced.

E. Denise Simmons criticized the scaled-down station designs at Union Square and East Somerville for having long ramps rather than elevators, saying they were not sufficient for accessibility.

[59] However, the 2009 draft environmental impact report replaced the Medford Hillside station with one slightly to the south at College Avenue, and deferred Mystic Valley Parkway to a future second phase.

[133] Some work like tree clearing, removal of old siding tracks, and geotechnical boring took place in the first half of 2018, while design of stations and other complex elements continued.

[145] (The work was originally expected to require closing the viaduct for 17 months – the third major disruption of Lechmere service since 2004 – causing opposition to the closure in Cambridge.

[151] The final concrete span of the Red Bridge viaduct was poured in December 2020, with ballast laid later that month and track laying starting in January 2021.

That month, MassDOT moved to return the contributions from Cambridge and Somerville, as the combination of construction being under budget and American Rescue Plan Act funds being received meant the municipal monies were not needed.

[166] Pre-revenue service, where trains on the Union Square Branch were operated on a revenue schedule but without passengers aboard, began on January 16, 2022.

[7] In August 2022, the MBTA indicated that the Medford Branch opening was delayed to late November 2022 due to several factors including additional testing of the traction power system.

[172] The Union Square Branch was closed from September 18 to October 10, 2023, during repairs to Squires Bridge, which carries the McGrath Highway over the tracks.

A tentative settlement reached with the property owner will also allow pedestrian connections to be built between East Somerville station and the Inner Belt area.

Along with Assembly station, it will increase the proportion of the city's population within walking distance of rapid transit from 15% to 85%; 192 acres (78 ha) of land (mostly near Union Square) were opened for redevelopment in parallel with the project.

[183] Transit construction can result in gentrification – and displacement in turn – as wealthier residents seek to live close to stations, though this varies substantially depending on other factors like development planning.

[187] The US2 mixed-use development in Union Square, which is primarily replacing light industrial buildings, is intended as an anti-gentrification measure by increasing housing supply and subsidizing nearby households and businesses at risk of displacement.