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. The 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 is 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.