Photo courtesy of the MBTA

The start of service is on the horizon for the most significant expansion of the MBTA’s core system in decades, but riders will need to continue their wait to learn when they’ll be able to ride the Green Line all the way into Medford.

MBTA officials said Thursday that work is almost complete on the long-anticipated Green Line Extension project, but the previous May 2022 target to open the larger of two lines remains up in the air. The single-stop Union Square Branch and a renovated Lechmere Station are set to open in March.

The second branch of the extension, which includes five total stops between East Somerville and Medford’s Tufts University, will be “not too far after that,” GLX Project Manager John Dalton said while presenting a slide that described the start of service on that segment as “under review.”

“We’re still kind of finalizing the schedule, getting that last bit of work done and getting the real testing and commissioning process off the ground,” Dalton told the MBTA Board. “We will soon be talking about when we will be getting Branch 2 in service.”

The Green Line Extension – which once appeared doomed before a turnaround brought it within reach – has been delayed several times. Last fall, when they postponed the Union Square Branch’s opening from December to March, T officials said they were not sure if they would meet the May target to open the rest of the extension to riders.

While the extension’s opening day is in flux, officials are confident they remain in position to complete the project within its $2.3 billion budget. The strong financial outlook spurred the T to reimburse Somerville and Cambridge for the $75 million the two cities collectively pledged toward the project at a time when its outlook was imperiled.

Union Square will be the first brand-new MBTA rapid transit stop to open since the Orange Line’s Assembly Station in 2014 and like that station has spurred a land rush and the development of tens of thousands of square feet of new commercial real estate. As a whole, the Green Line Extension represents the largest alteration to the core subway system since the 1980s, when the agency demolished elevated portions of the Orange Line between Chinatown and Forest Hills. The T dropped most of the infrastructure below ground and in 1987 opened nine new accessible stops