An MBTA Green Line train idles during a test run Feb. 3, 2022 at the platform at a brand-new Union Square station in Somerville, which is set to open in March as the first leg of the Green Line Extension project. Photo by Chris Lisinski | State House News Service / File

Mark your calendars now: March 21 is set to go down as the most significant Opening Day in 35 years.

Not for the Red Sox, of course, with regular-season baseball games imperiled by the ongoing lockout and a formal start date now anyone’s guess. But for the MBTA, that late-March Monday will kick off passenger service on the Green Line’s brand-new Union Square branch in Somerville, the first opening of a new line on the T’s core rapid transit network since 1987.

MBTA General Manager Steve Poftak announced Thursday that March 21 will feature a “grand opening ceremony” with revenue service beginning “some time mid- to late afternoon.”

Commuters will be able to board Green Line trains in Somerville for the first time ever, starting at Union Square and continuing through a newly constructed Lechmere Station, a reopened Science Park station and then onto the rest of the network.

“This is the T delivering on its commitment to get this open in March. We look forward to everyone who’s going to join us,” Poftak said. “There’s been so many people who have been involved in this project, not only from the beginning but from the moment it was essentially reborn.”

The upcoming opening will bring the smaller of the Green Line Extension’s two legs online. The project’s second branch features another five stops beyond Lechmere in Somerville and Medford, ending at College Avenue near Tufts University.

Work on that segment is close to complete, but officials have delayed the target date for completion several times. Last year, officials pushed the expected opening for the Medford branch from the end of 2021 to May 2022; in recent weeks, Poftak has said repeatedly that the May goal now appears in doubt.

While unveiling the start date for Union Square service on Thursday, Poftak said the other branch now does not figure to launch until “late summer.”

“We are still working on when the opening will be for the College Ave. branch, but we anticipate that opening in late summer,” he told the agency’s board of directors. “We will be coming back to you with a firmer date in the future.”

Extending the Green Line northward had been a driver of debate for decades, with roots in environmental mitigation required to offset impacts of the Big Dig. The project “essentially crashed and burned” in 2015 and 2016, as Poftak put it earlier this month, amid cost overruns and contract issues.

But the revival effort has been a success, and the T expects to wrap up work under the $2.3 billion budget and with enough room to reimburse Somerville and Cambridge for the $75 million they collectively pledged at a time when the extension was imperiled.

The last time the MBTA took such a significant step was in 1987, when the agency finished a massive project to demolish the elevated Orange Line and replace it with new infrastructure, including some underground stops.

“I was in high school then, so I don’t have a strong recollection of that particular opening,” Poftak said. “Nevertheless, it’s a really proud moment for the T, a really important moment for the region and a symbol that we can do big things, that we can get things done.”

Assembly Station, which came online on the Orange Line in 2014, is the most recent brand-new single subway stop to open.

Green Line Extension Gets Official Opening Day

by State House News Service time to read: 2 min