The Green Line Extension (GLX) was a construction project to extend the Massachusetts Bay Transportation Authority (MBTA) Green Line light rail system northwest into Somerville and Medford, two inner suburbs of Boston, Massachusetts. The project opened in two phases in 2022 at a total cost of $2.28 billion. Total ridership on the extension is estimated to reach 45,000 one-way trips per day in 2030.
The project begins at the north end of the Lechmere Viaduct, where the former ground-level Lechmere station was replaced by an elevated station on an extended viaduct. Two branches split north of Lechmere, with the Union Square Branch following the MBTA Commuter Rail Fitchburg Line right of way to Union Square station in Somerville. The Medford Branch follows the Lowell Line right-of-way to Medford/Tufts station with four intermediate stations. A new vehicle maintenance facility and storage yard are located in the Inner Belt District. The project includes a extension of the Somerville Community Path to North Point.
An extension of service beyond Lechmere was considered as early as 1922, with the first formal proposal in 1926. Despite appearing in other regional plans throughout the 20th century, the state did not commit to the project until 1990. Planning began in 2006 with a projected completion date of 2014; this was delayed to 2015, then to 2018. A groundbreaking was held in 2012, with several early construction elements over the next two years. The federal government committed $996 million of the $2.2 billion cost in 2014.
In 2015, with projected costs increased to $3.3 billion, the project was placed on hold. A revised plan, with more modest stations and other value engineering, was submitted in 2016 and approved in 2017. 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. Construction began in 2018, with the old Lechmere station closed in May 2020. The new Lechmere station and Union Square Branch opened on March 21, 2022, while the Medford Branch opened on December 12, 2022. The Community Path extension opened in June 2023.
Route and design
The GLX is primarily located in Somerville, northwest of downtown Boston, with its northernmost portion in Medford and its southernmost portion in the East Cambridge section of Cambridge. The GLX begins at the north end of the Lechmere Viaduct in East Cambridge, where a short elevated section formerly led to the surface-level Lechmere station at Lechmere Square. The new elevated GLX viaduct continues northwest for about , paralleling the O'Brien Highway. A new elevated Lechmere station is located on the viaduct on the east side of Lechmere Square. Two branches split at an aerial flying junction on the Red Bridge viaduct in the Inner Belt area.
The Medford Branch continues northwest to , with intermediate stations at , , , and . It runs in a below-grade cut along with the two-track Lowell Line and is operated as part of the E branch. An extension of the Somerville Community Path runs along the west side of the Medford Branch south of Lowell Street, with a high bridge carrying it above the Red Bridge viaduct to a new terminus in the Northpoint development. An additional extension from Medford/Tufts to is proposed, but it is not being constructed as part of the main GLX.
The Union Square Branch continues west to , with no intermediate stations. It runs at grade next to the two-track Fitchburg Line and is operated as part of the D branch. The vehicle maintenance facility (VMF) is located in the Inner Belt area adjacent to the MBTA Commuter Rail Maintenance Facility. It includes a four-track shop building, ten storage tracks with capacity for about 43 light rail vehicles (LRVs), a loop track, and a building for operator reporting and breaks. Yard leads connect the VMF to both branches near the Red Bridge flyover.
The GLX is fully grade separated, with no road grade crossings, though there are level crossings for passengers at East Somerville and Ball Square stations. Construction of the project included the replacement or modification of three rail bridges and seven road bridges. Maximum speed on the extension is , except for limits for the Union Square Branch at Red Bridge. As with the existing Green Line subway and D branch, the GLX has automatic block signaling with color light signals. The signal system is designed for 5-minute headways on each branch, with 2-minute headways through Lechmere.
Stations
The seven GLX stations are accessible; all have elevators between street level and platform level except East Somerville, which has a ramp from street level. The stations have fare vending machines, but not faregates. Most of the stations have island platforms wide and long – enough to fit three current LRVs or two future Type 10 LRVs – with provision to extend them to in the future. The platform at Lechmere is wide and long – enough to fit two current two-car trains – as it would be difficult to later extend the elevated platform. The platforms are high for accessible boarding on current LRVs, and can be raised to for future level boarding with Type 9 and Type 10 LRVs.
Stations have validators on the fare vending machines, with proof of payment enforcement in effect. When the station designs were simplified in 2016–17, faregates were removed from the plans. 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. However, delays to the AFC 2.0 system means the GLX opened prior to all-door boarding being implemented.
Station City Neighborhood(s) Service(s) Opening Cambridge Northpoint / East Cambridge D, E March 21, 2022 Union Square Branch Somerville Union Square / Ward Two D March 21, 2022 Medford Branch Somerville East Somerville E December 12, 2022 Winter Hill / Gilman Square E December 12, 2022 Magoun Square E December 12, 2022 Somerville / Medford Ball Square E December 12, 2022 Medford South Medford / Tufts University E December 12, 2022 Somerville West Somerville E Proposed
Prior proposals Early transit services
The Boston and Lowell Railroad (B&L) opened through South Medford, Somerville, and East Cambridge in 1835. Passenger service initially ran express between the two cities, but local stops were soon added. The Charlestown Branch Railroad opened through Somerville and Charlestown in 1842; it was soon extended as the Fitchburg Railroad. In 1870, the B&L built a cutoff from to Somerville Junction, which allowed Lexington Branch trains to enter Boston on the B&L. The Massachusetts Central Railroad began service in 1881; it used most of the 1870-built cutoff to reach Boston. It was leased in 1886 by the B&L, which was in turn leased by the Boston and Maine Railroad (B&M) in 1887 as its Southern Division. The Fitchburg was leased by the B&M in 1900 as the Fitchburg Division. By the early 20th century, the Southern Division had stations at Medford Hillside and Tufts College in South Medford; North Somerville, Somerville Junction, Winter Hill, and Prospect Hill in Somerville; and East Cambridge. Stations were located on the Lexington Branch at North Cambridge, West Somerville, and Somerville Highlands. The Fitchburg Division had stations at Cambridge, Somerville, and Union Square.
Horsecar service by the Cambridge Railroad in Somerville began in 1858 with lines from Union Square to Sullivan Square and to Elm Street (now Davis Square) via Somerville Avenue. The Cambridge Railroad was granted permission in 1860 to run tracks over the Craigie Bridge from East Cambridge to Boston, joining with its existing line on Cambridge Street from Harvard Square, though service did not begin until around 1862. The Somerville Avenue line was extended from Union Square to Boston in 1864, joining the existing line at Lechmere Square in East Cambridge. The Cambridge Street line was converted to electric streetcar service by Cambridge Railroad successor West End Street Railway in 1891; the Somerville Avenue line and an 1888-built line to Davis Square via Highland Avenue were electrified and extended to Clarendon Hill in 1893–1895.
The West End was leased to the Boston Elevated Railway (BERy) in 1897. The three streetcar lines running through Lechmere Square began running into the Tremont Street subway upon its September 3, 1898, opening. Slow and unreliable service on the surface streetcar lines resulted in congestion in the subway. The Lechmere Viaduct and Causeway Street elevated opened on July 1, 1912, creating a grade-separated route from Lechmere Square to the subway and bypassing the Cragie Bridge. However, the surface lines through Somerville continued to be unreliable, and multiple-car trains were needed to increase subway capacity. On July 10, 1922, the BERy opened Lechmere station as a transfer point. 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.
Early proposals
In 1907, the BERy proposed a branch of the Charlestown Elevated following Mystic Avenue from Sullivan Square to Medford Square. In 1922, with the downtown subway network and several radial lines in service, the BERy indicated plans to build three additional radial subways: one paralleling the Midland Branch through Dorchester, a second branching from the Boylston Street subway to run under Huntington Avenue, and a third extending from Lechmere Square northwest through Somerville. (The Dorchester line was eventually built as an extension of the Cambridge–Dorchester Tunnel later that decade, while a shorter Huntington Avenue subway opened in 1941.)
The Report on Improved Transportation Facilities, published by the Boston Division of Metropolitan Planning in 1926, was the first comprehensive transit plan for the Boston area since the 1890s. The core recommendation of the report was a conversion of the Tremont Street subway to Maverick Square–Warren Street, Brighton and Lechmere Square–Brigham Circle, Roxbury rapid transit routes. Several suburban rapid transit extensions were proposed as secondary priorities; one would run in a dedicated median in the Northern Artery from Lechmere to Winter Hill, alongside the B&M Southern Division tracks to Somerville Junction, then follow the cutoff (by then called the Fitchburg Cutoff) to North Cambridge. Several branches of that line were listed as possible extensions: North Cambridge to via the Lexington Branch, North Cambridge to via the Fitchburg mainline, and Somerville Junction to via the Southern Division and Woburn Branch.
Variations on this plan were proposed several times during the 1930s. A 1935 proposal by a citizen's group called for a subway extension from or surfacing in the North Station rail yards, then following the Southern Division and the Fitchburg Cutoff to Alewife Brook Parkway in North Cambridge. "High-speed trolley connections" would run to Arlington Heights and Belmont. A new expressway would have taken over the Causeway Street elevated and Lechmere Viaduct, then largely paralleled the new transit line to connect with the existing Mohawk Trail expressway at Alewife Brook Parkway. A 1938 proposal by Somerville mayor John M. Lynch called for an extension of the East Boston Tunnel from Bowdoin to the Lechmere Viaduct, a subway from Lechmere to Washington Street, and use of the Southern Division, Fitchburg Cutoff, and Lexington Branch to reach Arlington Center. A 1939 state resolve directed the Metropolitan District Commission to study that proposal.