Officials broke ground Monday on the long-delayed MBTA Green Line Extension into Somerville and Medford.

U.S. Transportation Secretary Elaine Chao was on hand, as were Gov. Charlie Baker and other state and local officials.

"I swore publicly that I would never come to another groundbreaking of the Green Line because I had already been to three," said U.S. Rep. Michael Capuano, who's also a former mayor of Somerville. "Today is different because this one is real. It’s been a long time coming."

Gov. Charlie Baker and other officials mark the groundbreaking of the Green Line Extension, on Monday in Somerville. (Courtesy of Rachel Mandelbaum for the Office of the Governor)

Somerville's current mayor, Joseph Curtatone, said he's elated the project is finally happening after years of uncertainty.

"The Green Line Extension project will add more than $3 billion of economic activity over the next two decades," Curtatone said. "Folks, I submit this is a damn good project and long awaited."

On Monday, the federal government released a second installment of $225 million for the estimated $2.3 billion project.

The 4.7-mile extension, which is slated to open in 2021, includes the development of six new stations, and the relocation of Lechmere Station in East Cambridge. It also includes a fleet of 24 new train cars.

With reporting by WBUR's Yasmin Amer