Green Line extension to Union Square will open March 21, MBTA says

The first part of the Green Line extension project is set to open in March.David L. Ryan/Globe Staff

The long-awaited Green Line extension to Union Square in Somerville will open for passenger service on the afternoon of March 21, MBTA General Manager Steve Poftak announced Thursday.

New stations at Lechmere and Union Square will welcome passengers that day, marking a major milestone for the Massachusetts Bay Transportation Authority, which has been discussing extending the Green Line for decades.

“It’s a really proud moment for the T, a really important moment for the region, and really a symbol that we can do big things and we can get things done,” Poftak said at a MBTA board meeting.

The timing of the opening of the separate and much longer Green Line extension branch to Medford, which includes five new stations, was tentatively scheduled for May but may not start carrying passengers until summer.

Both branches, which cost the MBTA a whopping $2.3 billion, have faced a seemingly endless series of delays dating back decades.

In 1990, the state promised to extend the Green Line to “Ball Square/Tufts University” as part of an agreement with the Conservation Law Foundation, a legal advocacy group, to mitigate the environmental impacts of the Big Dig, which buried Interstate 93 beneath the center of the city. The foundation sued the state in 2005, accusing it of stalling the project, and settled in 2007 when the government agreed to complete it by 2014.

An interior view of the newer Green Line train.David L. Ryan/Globe Staff

Further delays dragged the project into 2015 when the budget swelled to $3 billion. Massachusetts considered scrapping it entirely. The MBTA cut costs and eventually won approval from its then-oversight board to continue with the project in 2017.

The 4.7-mile extension to Somerville and Medford began in 2018 with the goal of beginning passenger service at the seven new stations by December 2021.

But last year, the MBTA pushed back the Union Square branch opening to March 2022, citing complications in building its substations, and the delivery date on the Medford branch, which terminates at Tufts University, citing supply chain impacts from the COVID-19 pandemic.

Despite the delays, transit advocates say they are thrilled to see the long-overdue project open for service next month.

The T expects shuttle buses to continue to replace subway service between Lechmere and North Station until the night before service begins on the new branch, said MBTA spokesperson Joe Pesaturo last month.

Test trains have been running along the tracks since Jan. 16.