Green Line Extension

Green Line Extension (GLX)

Company type	Public transit project
Industry	Public transportation
Founded	2012
Headquarters	Boston , United States
Area served	Boston, Somerville, Medford, Massachusetts
Products	Light rail transit extension
Parent	Massachusetts Bay Transportation Authority (MBTA)
Website	www.mbta.com/projects/green-line-extension-glx-project

The Green Line Extension (GLX) represents a significant MBTA initiative to expand Boston's Green Line light rail northwestward into the neighboring cities of Somerville and Medford. Completed in two stages throughout 2022, this $2.28 billion project covers a distance of and is projected to accommodate approximately 45,000 one-way trips daily by the year 2030.

Beginning at the northern terminus of the Lechmere Viaduct, the extension replaced the previous ground-level Lechmere station with a newly constructed elevated facility. From just north of Lechmere, two separate branches diverge: the Union Square Branch follows the Fitchburg Line corridor westward to Union Square station in Somerville, while the Medford Branch tracks alongside the Lowell Line northwestward to Medford/Tufts station, including four stops along the way. Additionally, the project established a new vehicle maintenance and storage yard within Somerville’s Inner Belt District and extended the Somerville Community Path by to reach North Point in Cambridge.

Although plans for extending beyond Lechmere date back to 1926 and were featured in various regional proposals during the twentieth century, official state backing was not secured until 1990. Formal design and planning efforts commenced in 2006 with an anticipated completion date set for 2014; however, this timeline was repeatedly delayed until 2018. Construction officially started in 2012 with preliminary work continuing through 2014, supported by a federal grant of $996 million toward the total estimated cost of $2.2 billion.

By 2015, escalating expenses pushed projected costs up to $3.3 billion, resulting in a temporary halt to progress. A revised plan focusing on cost-saving measures gained approval in 2017, followed by the award of a $1.08 billion design-build contract later that year. This contract included optional features such as platform shelters and an expanded maintenance facility. Work resumed in 2018, leading to the closure of the original Lechmere station in May 2020. The new Lechmere station and Union Square Branch began service on March 21, 2022, with the Medford Branch starting operations on December 12, 2022. The Community Path extension opened for public use in June 2023.

Route and design

The GLX primarily traverses Somerville, located northwest of Boston, extending into Medford as well as East Cambridge within Cambridge. Its origin point is at the northern end of the Lechmere Viaduct in East Cambridge, where an elevated viaduct replaces the former surface-level Lechmere station situated at Lechmere Square. This elevated structure runs roughly northwest alongside O'Brien Highway, featuring a newly constructed elevated Lechmere station positioned on its eastern side. From this location, two branches split at an aerial flying junction located on the Red Bridge viaduct within Somerville’s Inner Belt area.

The Medford Branch extends approximately northwest to , stopping at four intermediate stations: , , , and . This segment runs below ground adjacent to the dual-track Lowell Line and operates as part of the E branch. Included with this branch is an extension of the Somerville Community Path along its western flank south of Lowell Street, incorporating a high-level bridge over Red Bridge viaduct connecting to the NorthPoint development. Although there is a proposed further extension from Medford/Tufts to , this section is not included within the primary GLX project.

The Union Square Branch travels about westward to , without any intermediate stops. It functions at-grade alongside the two-track Fitchburg Line corridor as part of the D branch.

A vehicle maintenance facility (VMF) situated within Inner Belt near the existing MBTA Commuter Rail Maintenance Facility comprises a four-track servicing shop, ten storage tracks capable of housing around 43 light rail vehicles (LRVs), a loop track for continuous movement, and an operator building. Connections from this yard lead to both branches close to the Red Bridge flyover.

The GLX employs full grade separation, thereby removing road-rail crossings but allowing passenger level crossings specifically at East Somerville and Ball Square stations. The project involved replacing or altering three rail bridges along with seven road bridges. Maximum operational speeds reach , except for reduced limits of on the Union Square Branch near Red Bridge. The signaling system utilizes automatic block signaling with color light signals designed to support headways of five minutes per individual branch and as frequent as two and a half minutes through Lechmere station.

Stations

The seven GLX stations provide full accessibility, with elevators linking street level to platforms at all locations except East Somerville, which is equipped with a ramp instead. Each station is outfitted with fare vending machines but does not include faregates. The majority feature island platforms ranging from 20 to 22.5 feet in width and approximately 225 feet in length, designed to accommodate three of the current light rail vehicles (LRVs) or two future Type 10 LRVs, with the capability to extend platforms up to 300 feet. Lechmere station’s platform is notably larger—measuring between 32 and 35 feet wide and extending 355 feet—to support two two-car trains simultaneously, as future expansion there poses significant challenges. Platform heights are set at 8 inches for compatibility with existing LRVs, but can be adjusted to 14 inches to enable level boarding for upcoming Type 9 and Type 10 LRVs.

Fare validators are integrated into the vending machines, supporting a proof-of-payment system. Initial design revisions during 2016–17 eliminated faregates to simplify station layouts. The MBTA had planned for full implementation of the AFC 2.0 system (which replaces the CharlieCard) by the time GLX opened, allowing passengers to board through all doors by tapping farecards; however, delays meant that GLX began operation prior to this all-door boarding functionality becoming available.

Station	City	Neighborhood(s)	Service(s)	Opening
REDIRECT Template:Boston transportation system	Cambridge	Northpoint / East Cambridge	D, E	March 21, 2022
Union Square Branch
REDIRECT Template:Boston transportation system	Somerville	Union Square / Ward Two	D	March 21, 2022
Medford Branch
REDIRECT Template:Boston transportation system	Somerville	East Somerville	E	December 12, 2022
REDIRECT Template:Boston transportation system		Winter Hill / Gilman Square	E	December 12, 2022
REDIRECT Template:Boston transportation system		Magoun Square	E	December 12, 2022
REDIRECT Template:Boston transportation system	Somerville / Medford	Ball Square	E	December 12, 2022
REDIRECT Template:Boston transportation system	Medford	South Medford / Tufts University	E	December 12, 2022
REDIRECT Template:Boston transportation system	Somerville	West Somerville	E	Proposed

Early transit services

The Boston and Lowell Railroad (B&L) commenced operations through South Medford, Somerville, and East Cambridge in 1835. Initially providing express passenger service between these cities, it soon incorporated local stops. The Charlestown Branch Railroad, which opened in 1842 running through Somerville and Charlestown, was later extended as part of the Fitchburg Railroad. In 1870, the B&L constructed a cutoff line from an unspecified location to Somerville Junction, allowing trains on the Lexington Branch to access Boston via the B&L tracks. The Massachusetts Central Railroad started service in 1881 utilizing much of this cutoff route to reach Boston. The B&L was leased by the Boston and Maine Railroad (B&M) in 1887 as its Southern Division; similarly, B&M leased the Fitchburg Railroad in 1900 as its Fitchburg Division. By the early twentieth century, stations on the Southern Division included Medford Hillside, Tufts College, North Somerville, Somerville Junction, Winter Hill, Prospect Hill, and East Cambridge. The Lexington Branch featured stops at North Cambridge, West Somerville, and Somerville Highlands. Stations on the Fitchburg Division included Cambridge, Somerville, and Union Square.

Horsecar lines operated by the Cambridge Railroad began serving Somerville in 1858 with routes running from Union Square to both Sullivan Square and Elm Street (present-day Davis Square) via Somerville Avenue. In 1860, the Cambridge Railroad received authorization to lay tracks over the Craigie Bridge connecting East Cambridge to Boston; this complemented its existing Harvard Square line and service began around 1862. By 1864, the line along Somerville Avenue extended from Union Square all the way into Boston via a connection at Lechmere Square. Under its successor company West End Street Railway in 1891, the Cambridge Street line was converted from horsecar to electric streetcar service; electrification and route extensions including those along Somerville Avenue and a branch to Davis Square were completed by 1895.

In 1897, the West End Street Railway was leased by the Boston Elevated Railway (BERy). Starting September 3, 1898, three streetcar lines passing through Lechmere Square were routed into the newly opened Tremont Street subway. Due to ongoing surface streetcar congestion issues, grade-separated infrastructure was developed: notably the opening of the Lechmere Viaduct and the elevated Causeway Street line on July 1, 1912—these bypassed Craigie Bridge. Despite such improvements, surface streetcars remained prone to delays; subway capacity required multiple-car trains. On July 10, 1922, BERy inaugurated Lechmere station as a transfer hub where riders from Harvard Square and Clarendon Hill lines could connect with two- or three-car subway trains—marking the introduction of rapid transit-style service at Lechmere.

Early plans

In 1907, the BERy suggested constructing a branch of the Charlestown Elevated along Mystic Avenue, connecting Sullivan Square to Medford Square. By 1922, after several radial lines were already in operation, BERy envisioned three new radial subway routes: one paralleling the Midland Branch through Dorchester; another branching off from the Boylston Street subway beneath Huntington Avenue; and a third extending northwest from Lechmere Square through Somerville. The Dorchester route was eventually realized as an extension of the Cambridge–Dorchester Tunnel, while the Huntington Avenue subway commenced service in 1941.

The 1926 Report on Improved Transportation Facilities by Boston’s Metropolitan Planning Division advocated for transforming the Tremont Street subway into rapid transit corridors reaching Maverick Square–Warren Street and Lechmere Square–Brigham Circle. Proposed suburban rapid transit expansions included a line running in the Northern Artery median from Lechmere to Winter Hill, then following B&M Southern Division tracks to Somerville Junction, and continuing along the Fitchburg Cutoff to North Cambridge. Potential branches comprised routes from North Cambridge to via Lexington Branch, from North Cambridge to via the Fitchburg mainline, and from Somerville Junction to utilizing Southern Division and Woburn Branch tracks.

Throughout the 1930s, alternative concepts surfaced: a 1935 citizen-submitted plan proposed extending a subway from or surfacing near the North Station rail yards, continuing along Southern Division and Fitchburg Cutoff corridors to Alewife Brook Parkway, integrating an expressway parallel to this transit line. In 1938, Somerville’s mayor John M. Lynch recommended extending the East Boston Tunnel from Bowdoin to Lechmere Viaduct and leveraging existing rail corridors for service reaching Arlington Center. A legislative resolution in 1939 mandated further examination of this proposal.

During the Great Depression and with growing automobile competition along the Northern Artery, transit services in Somerville deteriorated. Multiple stations were shuttered between 1927 and the early 1940s. The Harvard Square–Lechmere route (now route ) transitioned to trackless trolley operation in 1936; by 1940, six streetcar lines remained active within Somerville. Routes 87 and 88 converted to trackless trolley in 1941; routes 89 and 101 followed suit in 1947. Service on Fellsway routes persisted until 1955.

In 1943, a state-appointed commission under Arthur W. Coolidge developed a plan for Boston’s transit future. Its preliminary report in 1945 proposed nine suburban rapid transit extensions along existing rail corridors, closely reflecting earlier schemes. The extension from Lechmere to Woburn was designed with an elevated structure north of Lechmere and a short subway section near Winchester Center to eliminate grade crossings. Annual ridership was projected at 11.8 million. The final report issued in 1947 extended this line an additional two miles to North Woburn with an estimated cost of $3.627 million (in 1941 dollars). This commission also paved the way for BERy’s transition to public ownership as the Metropolitan Transit Authority (MTA) later that year.

By 1958, B&M discontinued service at Medford Hillside, Tufts College, and North Somerville stations due to diminishing passenger numbers. Between 1959 and 1963, routes shifted from trackless trolley to diesel bus operation, establishing buses as the dominant transit mode in Somerville. The North Terminal Area Study of 1962 recommended abandoning the elevated segment between Lechmere and North Station after suburban rapid transit proved inadequate following previous conversions; it suggested rerouting the Main Line (now Orange Line) along B&M Western Route with a branch extending toward Woburn or Arlington via Southern Division tracks.

MBTA era

The Massachusetts Bay Transportation Authority (MBTA) was formed in 1964 with the objective of broadening the MTA’s functions by providing subsidies for suburban commuter rail and developing rapid transit extensions to supplant certain commuter rail routes. In 1965, the Tremont Street subway along with its connecting lines was renamed the Green Line. The 1966 Program for Mass Transportation emphasized an approximate extension from Lechmere to Washington Street, coinciding with the reduction of New Hampshire Division passenger service to Washington Street. A subsequent phase suggested further extending Green Line service to either Mystic Valley Parkway or West Medford.

According to the 1972 Boston Transportation Planning Review, a Green Line extension from Lechmere to was considered a low priority, with an estimated cost of $26 million in 1966 dollars. The 1974 plan upheld this low priority status, largely due to the upcoming Red Line Northwest Extension which included Davis station, opened in 1984 as Somerville’s inaugural MBTA subway stop. Proposals for extensions reaching Tufts University surfaced in both the 1978 and 1983 program updates.

In 1980, the MBTA launched a study focused on the "Green Line Northwest Corridor," aimed at replacing the Causeway Street elevated structure, relocating Lechmere station, and extending service beyond it. Reports from evaluations conducted in 1981 and 1984 recommended further investigation into extending service to Tufts and , alongside consideration of a busway alternative. Plans developed in the late 1980s intended to move Lechmere station eastward of O'Brien Highway and add new storage and maintenance facilities; however, financial limitations prevented these plans from advancing.

Early planning

In 1991, as part of a settlement stemming from litigation with the Conservation Law Foundation concerning emissions from the Central Artery/Tunnel Project, the state pledged support for various transit initiatives. Among these was a "Green Line Extension To Ball Square/Tufts University," with a targeted completion date of 2011. While other projects such as the Old Colony Lines restoration progressed, the Green Line Extension (GLX) experienced delays. The 2003 Program for Mass Transportation assigned medium priority to both Green and Blue Line extensions toward West Medford, noting that the Blue Line option was more expensive due to tunneling requirements. By 2005, with no substantial advancement on GLX, Somerville and CLF filed suit against the state for violating the 1991 agreement.

A revised agreement in May 2005 incorporated enhancements to the Fairmount Line and Framingham/Worcester Line, eliminated plans for the Red-Blue Connector and Arborway restoration, and redefined GLX to feature branches reaching Union Square and Medford Hillside. The project cost was projected at $559 million (2005 dollars). Medford expressed support for extending service to Medford Hillside but showed limited enthusiasm for West Medford.

The Beyond Lechmere Northwest Corridor Study conducted in 2005 evaluated multiple GLX options including branches to West Medford and Union Square, bus rapid transit (BRT), and commuter rail shuttle services. The highest-ranked alternatives were Green Line extensions to West Medford ($390 million), combined West Medford and Union Square ($438 million), and Union Square paired with BRT service to West Medford ($340 million).

Following a court settlement in November 2006, GLX completion was scheduled for 2014, receiving Environmental Protection Agency approval in July 2008. Proposed stations announced in May 2008 comprised Union Square, Brickbottom, Gilman Square, Lowell Street, Ball Square, Medford Hillside, and Route 16. Maintenance and storage facilities were planned at the former Yard 8 location.

While Somerville strongly endorsed the project as a catalyst for development and a remedy for historic transportation disparities, suburban Medford exhibited comparatively less support. Cost assessments conducted in 2008 concluded that tunnel alternatives—such as tunneling from Ball Square to , constructing an underground Union Square station, or tunneling portions of the Medford Branch—were not financially justified.

Environmental review

In October 2006, the Massachusetts Executive Office of Transportation and Public Works submitted an Expanded Environmental Notification Form (EENF) that designated the Green Line extension, including branches to Medford and Union Square, as the preferred option, requesting approval to prepare a single environmental impact report. However, by December, the Secretary of Environmental Affairs mandated separate draft (DEIR) and final (FEIR) environmental impact reports. The DEIR development commenced in September 2007.

Released in October 2009, the DEIR excluded West Medford as a terminal point because of the substantial expenses involved in altering bridges spanning the Mystic River and Mystic Valley Parkway, along with safety issues related to grade crossings. To enhance accessibility for northern neighborhoods, the Medford Hillside station was substituted with a College Avenue station. The favored plan featured branches extending to Route 16 and Union Square; nevertheless, budget limitations postponed the Route 16 branch to a subsequent phase, with College Avenue serving as the temporary endpoint.

The FEIR, published in June 2010, projected project expenses at $845 million (in 2009 dollars). A significant revision involved relocating the maintenance facility approximately a quarter-mile eastward, adjacent to the MBTA Commuter Rail Maintenance Facility, following objections from local residents. In July 2010, MassDOT announced a postponement of the project’s completion date to October 2015.

In November 2010, the MBTA finalized an agreement with Pan Am Railways and NorthPoint developers exchanging land necessary for constructing the new Lechmere station in return for redevelopment rights at the current site. This agreement also granted MBTA trackage rights on the Worcester Branch and in New Hampshire; it was completed by March 2011.

An additional postponement was declared in August 2011, delaying service commencement until late 2018–2020 due to issues acquiring land and concerns over cost management and financing. Because this delay extended beyond the legal deadline of December 31, 2014, MassDOT implemented interim measures to reduce emissions. These included increasing midday Green Line service frequency to Lechmere and enhancing several bus routes (80, 88, 91, 94, 96), acquiring hybrid vehicles for The Ride paratransit service, and constructing new parking garages at Salem and Beverly stations.

Funding and contracts

By June 2012, the project entered the preliminary engineering stage under the Federal Transit Administration (FTA) New Starts program, enabling access to federal funds. The estimated budget was $1.34 billion, which incorporated $200 million in finance charges. In July 2012, the FTA issued a Finding of No Significant Impact (FONSI), permitting the MBTA to initiate bidding for the Phase 1 construction contract. This $13 million contract was awarded to Barletta Heavy Division in December through a traditional design–bid–build procurement process.

In August 2012, an agreement between Somerville city officials, MassDOT, and MBTA set the Union Square Branch opening for early 2017 with construction beginning in 2014. By September 2013, MassDOT had awarded a $393 million contract with a duration of 51 months for Phase 2/2A construction—covering Lechmere station, Union Square Branch, and part of the Medford Branch—aiming for station openings by early 2017. MassDOT planned to pursue $557 million in federal funding for Phase 3 (maintenance facility) and Phase 4 (remaining Medford Branch), targeting completion by either 2019 or 2020.

In April 2014, Governor Deval Patrick signed a $12.8 billion bonding bill that allocated up to $1.3 billion specifically for the Green Line Extension. That same month saw MassDOT include a $39 million extension of the Somerville Community Path within the project scope. May brought approval of a $118 million contract with CAF USA to manufacture 24 Type 9 LRVs, expanding rolling stock capacity for GLX operations. By September 2014, project costs had escalated to $2.3 billion—comprising $1.6 billion in direct expenses plus contingency funds and finance charges—due to added elements such as the Community Path extension and supplementary viaduct construction.

In December 2014, the FTA committed $996 million toward covering part of the total $2.3 billion cost; state funds were expected to finance the remainder. The Full Funding Grant Agreement (FFGA) was executed in January 2015.

Escalation in expenses and project redesign

Before securing financing, internal cost projections showed significant discrepancies. In December 2013, the construction manager White Skanska Kiewit (WSK) projected construction expenses at $1.17 billion, with an overall project budget of $2.35 billion. A month later, the project management team HDR/Gilbane estimated construction costs at $853 million and a total budget of $1.83 billion. By June 2014, the MBTA allocated $1.99 billion (rising to $2.3 billion when including financing costs) based on HDR/Gilbane’s numbers; these figures formed the basis for the FFGA.

By August 2015, the MBTA disclosed that anticipated costs had surged by $700 million to $1 billion beyond the initial $1.99 billion estimate. Several segments experienced steep increases, notably Phase 2/2A, which escalated from $387 million to $898 million. Critics such as the CLF linked these overruns to the CM/GC procurement method, which permitted WSK to set subcontractor prices without traditional competitive bidding. Consequently, in December 2015, the MBTA terminated contracts with WSK and three additional companies; ongoing work proceeded but no new agreements were issued. Considerations for cancellation and cost reductions included dropping the Union Square Branch.

In May 2016, boards of MassDOT and MBTA endorsed a cost-saving redesign through value engineering. Station designs were simplified to mirror Green Line D branch surface stops by removing features like canopies, faregates, escalators, and certain elevators. Two planned bridge replacements were eliminated, while two others were downsized by constructing new spans behind existing supports. The vehicle maintenance facility’s capacity was halved to accommodate 44 LRVs instead of 88. Additionally, the southern portion of the Community Path was truncated, decreasing street access points—measures intended to realign total expenditures with the $2.3 billion FFGA target.

In August 2016, the FTA indicated its conditional support but cautioned that delays could drive up expenses. A new project manager was appointed in November 2016. By December, funding agreements secured contributions of $25 million from Cambridge and $50 million from Somerville; concurrently, the MBTA announced a revised operational date set for 2021. The FTA approved the updated cost projections in April 2017, and federal funding was first disbursed in July 2017.

Revised contract

In February 2017, three consortiums were shortlisted for the updated design-build contract, with a final request for proposals issued by the MBTA in May. Submissions were required to remain within an affordability ceiling of $1.319 billion to maintain total project costs under $2.3 billion. Six optional add-ons—station canopies, extra elevators, public art installations, an additional Community Path street connection, full extension of the Community Path, and enhancements to the vehicle maintenance facility—were included contingent on budget and timeline feasibility. In August 2017, construction manager CH2M was dismissed due to a potential conflict of interest after its acquisition by Jacobs Engineering Group, which was also bidding on the contract.

Proposals were submitted in September 2017; by October two met the affordability criteria. On November 17, 2017, MBTA awarded GLX Constructors—a partnership among Fluor Enterprises, Middlesex Corporation, Herzog Contracting Corporation, and Balfour Beatty Infrastructure—the contract to deliver the project. Their base bid stood at $954 million including all six optional elements; the final contract amounting to $1.08 billion incorporated contingency reserves. The contract was formally granted on November 20, 2017.

Certain cost-reduction design choices drew criticism from community members and local leaders. E. Denise Simmons voiced opposition to scaled-back station designs at Union Square and East Somerville that replaced elevators with extended ramps due to accessibility concerns. The Community Path extension’s width of 10 feet fell short of the federal minimum standard of 11 feet; Friends of the Somerville Community Path cautioned this could pose safety risks for cyclists unable to overtake safely.

Extension of Mystic Valley Parkway

The 1991 agreement initially identified "Ball Square/Tufts University" as the terminus for the GLX; this was later revised in 2005 to "Medford Hillside". By 2008, proposals included stations at Medford Hillside (located between College Avenue and Winthrop Street) and near Route 16/. However, the 2009 draft environmental impact statement substituted Medford Hillside with a station slightly further south near College Avenue and postponed Mystic Valley Parkway station to a subsequent phase. During public consultations in 2010–2011, some residents from Medford and Somerville expressed concerns about this alteration, contending that College Avenue lies outside Medford Hillside and emphasizing the necessity of Mystic Valley Parkway station to fulfill project commitments. In January 2012, MassDOT reaffirmed its stance on the GLX layout, receiving backing from regulatory bodies including MassDEP.

Plans for constructing Mystic Valley Parkway station were later included in Boston MPO funding allocations for 2016–2020, scheduled after completion of the main line; nevertheless, budget overruns led to reallocation of these funds. Some environmental assessments persisted, with a project change notice issued in October 2017.

Additionally, local authorities have suggested extending the Union Square Branch further to . The design of Union Square station allows for this potential expansion without restrictions.

Preliminary work

Phase 1 focused on rebuilding two railroad bridges—one over Harvard Street in Medford and another spanning Medford Street in Somerville—to accommodate Green Line tracks, alongside demolishing an obsolete MBTA building at 21 Water Street in Cambridge to clear space for the new Lechmere station. Construction officially commenced with a groundbreaking event at the Medford Street bridge on December 11, 2012. Barletta Heavy Division received the notice to proceed on January 31, 2013, initiating construction activities in March. The demolition of the 21 Water Street structure was completed in August 2014. Although originally targeted for early 2015 completion, Phase 1 concluded in October 2015.

The contract for Type 9 LRVs was awarded to CAF in September 2014. The initial vehicle arrived in March 2018 and began service by December that year. The remaining 23 LRVs were delivered between 2018 and 2020, entering operation from 2019 through 2021.

Primary construction

Early construction efforts, including building piers for the Red Bridge viaduct, took place during 2014–15 before work was paused. In December 2017, the FTA provided $100 million in funding, enabling MBTA to advance main construction. Activities at the start of 2018 included clearing trees, removing existing tracks, and conducting geotechnical borings while station designs progressed. A formal groundbreaking ceremony occurred on June 25, 2018. Subsequent tasks involved constructing viaducts, installing drainage systems, building retaining and noise barriers; temporary relocation of Lowell Line tracks was finalized by October 2018. By year-end, station designs had reached approximately 60% completion.

The Homans Building near Gilman Square was demolished in April 2019. Construction accelerated mid-year with annual expenditures increasing from $170 million to $400 million. By October 2020, four stations had achieved full design status; overall project completion reached 20% by November 2019. Viaduct spans were installed and commuter rail tracks shifted eastward to enable Green Line track and station construction.

The project required temporary shutdowns of three road bridges—Broadway, School Street, Medford Street—and one underpass at Washington Street for reconstruction purposes. Most closures occurred between March and May 2019; reopening phases extended from May 2020 through December 2022. In May 2020, Green Line service was truncated from Lechmere to North Station to connect the Lechmere Viaduct with the new GLX viaduct—a disruption initially planned for seventeen months but extended to twenty-two months due to delays. Demolition of the old northern section of Lechmere Viaduct began June 2020; simultaneously, rehabilitation work on the main viaduct segment proceeded.

By October 2020, more than half of the construction was complete. Platform installation intensified late that year with steel canopies and structural elements underway at several stations. The final concrete span of Red Bridge was poured in December; track installation commenced January 2021. The Union Square Branch opening was originally scheduled for October 2021 with Medford Branch following in December. By June 2021, approximately sixty-five percent of track had been laid alongside eight out of nine signal houses.

In June 2021, MBTA announced additional delays partly attributed to COVID-19 impacts: Union Square Branch's opening shifted to December 2021 and Medford Branch's to May 2022. Testing on Union Square Branch began August 2021; testing on Medford Branch started early 2022 coinciding with the Community Path extension opening. Operational adjustments included extending the D branch service to North Station and rerouting B and C branches as part of GLX integration in October 2021. Union Square Branch opening was postponed again to March 2022 due to substation complications; train testing initiated December 2021.

By November 2021, overall project progress reached eighty-five percent completion. MassDOT refunded municipal contributions from Cambridge and Somerville as construction costs came under budget and federal American Rescue Plan funds were secured. Testing on Medford Branch commenced May 2022 following Union Square Branch's March opening. The total estimated project cost stands at $2.28 billion: $996 million sourced federally and $1.28 billion from state and other funding streams. Expected daily ridership by the year 2030 is projected around forty-five thousand passengers.

Operations planning and commencement

The initial configuration paired the Union Square Branch with the E Branch and the Medford Branch with the D Branch. However, in April 2021, the MBTA revised this arrangement, assigning the D Branch to serve Union Square and the E Branch to Medford/Tufts. This decision was influenced by both branches’ connections to the Longwood Medical Area, with the D Branch matched to the shorter Union Square Branch due to its longer western extension. Initially, Union Square was served by the E Branch.

Pre-revenue operations on the Union Square Branch began on January 16, 2022. The MBTA announced that Lechmere and Union Square stations would open on March 21, 2022, while Medford's opening was postponed until late summer. Service at Union Square started as planned. In August 2022, Medford’s launch was further delayed to late November because of additional traction power testing requirements. The Union Square Branch was closed from August 22 through September 18 for final integration tasks, removal of speed restrictions on the Lechmere Viaduct, and demolition of the Government Center Garage. During this interval, trains from the D and C branches operated to Union Square while service on the E branch was suspended; since September 19, service to Union Square has primarily been provided by D branch trains. The Medford Branch officially opened on December 12, 2022.

In February 2023, Somerville secured a lease for the Community Path from the MBTA, which subsequently opened on June 10, 2023. The Union Square Branch underwent a closure from September 18 to October 10 for repairs to Squires Bridge spanning McGrath Highway. This closure was both postponed and shortened from an initially planned July–August timeframe following public opposition.

By June 2023, the MBTA had imposed temporary speed restrictions on GLX tracks—reducing speeds down to —due to identified track gauge irregularities, which expanded to by September. These speed limitations were removed in October. Later that month, MBTA revealed it had been aware of the narrow gauge issue since April 2021; remediation involves regauging approximately half of the Union Square Branch track and 80% of Medford Branch track by adjusting tie plates. Repair work commenced on November 27, 2023 but was not completed by the targeted December 10 deadline.

In December 2023, the MBTA Board authorized property acquisition to expand maintenance facilities accommodating future two-car Type 10 vehicles. A preliminary agreement with the property owner will also facilitate pedestrian access between East Somerville station and the Inner Belt area.

Economic effects

The GLX is projected to generate substantial economic advantages for Somerville by increasing residents within walking distance of rapid transit from roughly 15% to approximately 85%. Nearly of land near Union Square became available for redevelopment concurrent with the project. While transit expansions often accelerate gentrification and displacement as higher-income residents relocate closer to stations, outcomes depend heavily on local development policies. The opening of Davis station in 1984 previously spurred gentrification and elevated housing costs in Davis Square.

Displacement concerns surfaced early; a 2014 study estimated that between 740 and 810 households along the GLX corridor were vulnerable due to rising rents—up to 475 attributed to apartment-to-condo conversions and about 245 related to expiring housing subsidies. Increases in property taxes were not expected to be a major displacement factor. To alleviate pressures, an estimated addition of between 6,300 and 9,000 housing units would be necessary by 2030.

Union Square and East Somerville stations were forecasted to experience the steepest rent hikes—up to 67%. The US2 mixed-use development in Union Square aims to mitigate gentrification effects by expanding housing availability and providing subsidies for at-risk households and businesses. By late 2014, both residential and commercial rents in Union Square were rising at rates exceeding regional averages.

A Title VI analysis conducted in May 2021 determined that benefits from the project would disproportionately favor nonminority and non-low-income populations due to demographic shifts since initial planning; nonetheless, all groups would experience improved service. In response, MBTA preserved bus route service , which had been slated for discontinuation with GLX’s launch, as part of a comprehensive bus network redesign. By 2021, home sale prices in Somerville had increased by 40% since 2016. Once known as an affordable area favored by artists compared to Boston and Cambridge, Somerville now confronts a shortage of affordable studio spaces partly linked to GLX-driven gentrification.

External links

• Official MassDOT page for the Green Line Extension project
• MBTA's dedicated Green Line Extension project site
• Somerville Transportation Equity Partnership (STEP) organization
• 2014 visualizations: Medford Branch animation, Union Square Branch animation