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The leader of the MBTA’s Green Line Extension project told T overseers Monday that the trolley service extension is about 60 percent designed and said he has not seen signs of possible snags on the horizon.

John Dalton, the Green Line Extension project manager, also said that construction work is beginning to pick up ahead of the planned December 2021 grand opening.

Dalton said there are about 320 people working full-time on the roughly $2.3 billion, 4.7-mile light rail extension from Cambridge to Medford – 95 from the MBTA and its consultants and another 225 from the company hired to design and build the new line.

“That number will grow,” Dalton said. “We’ll see that number grow, certainly, over the next six to 12 months.”

Dalton said the bulk of the construction-related work taking place now is related to drainage infrastructure, retaining walls and noise barriers. Demolition of buildings will begin later this month, he said, and the impacts of the construction will soon become more evident.

The Broadway Bridge will close for 12 months starting in late March, and the Washington Street bridge will be closed from April until November to accommodate Green Line Extension construction, Dalton said.

Asked by T Fiscal and Management Control Board member Brian Shortsleeve if he’s seen any impediments to the project’s schedule, Dalton said no.

“Knock on wood, the weather has been cooperating. We have three winters between now and revenue service, so that’s always an unknown,” he said. “At this time, I can’t say anything is a red flag.”

After halting the trolley extension project in 2015 after its estimated costs had ballooned to $3 billion, MassDOT okayed a scaled-down $2.289 billion version of the Green Line Extension in 2016. It is expected to serve about 50,000 riders.