When completed, the Green Line would be extended from a relocated Lechmere Station in East Cambridge to Union Square in Somerville and College Avenue in Medford, a 4.7-mile aboveground project, according to the MBTA and Federal Transit Administration.

The commitment marks a major turning point for a frequently delayed expansion that advocates say will spur economic development, transform communities, and give commuters a one-ride transit option from as far away as the Tufts Medford campus into downtown Boston. The first portion of the expansion is now scheduled to be completed by 2017, officials said, and the remainder by 2020.

The federal government this week pledged nearly $1 billion to help finance the expansion of the Green Line from Cambridge into Somerville and Medford, a major boost for the long-promised transit project.


In endorsing the project, the federal government — which is paying more than 40 percent of the cost — noted the extension will introduce rail service to communities that do not have it.

“The state is positioned to do its part, the federal government is positioned to do its part, and there ought to be no obstacle to getting this done,” Governor Deval Patrick said Tuesday.

The MBTA estimated the expansion would create about 140 design jobs, 700 construction jobs, and 140 jobs in ongoing operations.

Mayor Joseph A. Curtatone of Somerville said the extension will be a boon for the neighborhoods and will draw more people to the region. “We know the impact the Green Line will have to improve our quality of life in the region and to help Somerville reach its goals,” he said.

Ken Krause, of the Medford Green Line Neighborhood Alliance, said the expanded rail service has the power to transform the city of Medford. “It’s going to provide much more frequent, reliable, cleaner, and accessible transit service to communities that really need it,” he said.


In a commitment letter released Tuesday, the FTA estimated the total Green Line extension project would cost just under $2.3 billion. After the federal government’s contribution of $996 million of competitive “New Start” grant funds, the remainder of the funding will come from the state.

Stephanie Pollack, the associate director of The Dukakis Center for Urban and Regional Policy at Northeastern University, said that if the federal transit agency had refused to endorse the project, the expansion could have been put off even longer as officials sought enough state funding for the entire project — likely by delaying or shelving other plans.

“This marks a huge milestone toward making the Green Line extension a reality and having a substantial federal investment in it,” Pollack said.

Though community and transit advocates have pushed for the expansion for decades, the project has long been beset by delays. The state committed to the Green Line extension in 1990, as a measure to offset the Big Dig’s environmental impact. The deadline set for the extension was 2011.

In 2005, the Conservation Law Foundation filed a lawsuit accusing the state of failing to meet its commitment. A settlement forced the state to set a deadline to complete the extension by 2014.

But state officials later said the extension would be delayed until 2015. They then pushed the completion date to 2018 and eventually 2020.

Under the latest timeline, the new Lechmere Station and Union Square and Washington Street stations would be open by 2017, according to MBTA spokesman Joe Pesaturo. The Gilman Square, Lowell Street, Ball Square, and College Avenue stations would be open by 2020, officials said.


Construction began in April 2013 to widen rail bridges along the extension route and demolish MBTA-owned facilities on Water Street in Cambridge. Preliminary work for the Lechmere, Union Square, and Washington Street stations began this fall, and more substantial work on those stations is scheduled to happen early next year.

US Representative Michael E. Capuano, who pushed for federal funding, said the project “rises above all others” because the state is legally bound to finish it under the terms of the lawsuit. “This is a legal commitment,” he said. “This is not just another project that we want to do.”

The federal funding agreement will be reviewed by Congress within the next 30 days before it becomes official, though the review is largely seen as a formality.

Transportation administration officials said they cannot comment on the project during the review period. In a Dec. 1 letter, the federal agency said it had reviewed the MBTA’s financial status and concluded the regional transit agency has the financial strength to pay for the remaining share of the project.

“The project cost is reasonable if the MBTA diligently follows its risk and contingency management plans and procedures,’’ wrote Therese W. McMillan, the acting administrator of the FTA. Analysts had concluded “that the MBTA has the requisite financial capacity to complete this project and continue to operate and maintain the existing system,” McMillan wrote.


The FTA said it endorsed the extension because it introduces rail service to communities, allows for a one-ride service to an estimated 342,000 jobs in downtown Boston, and helps improve air quality in the region by reducing the use of cars.

Beverly A. Scott, the general manager of the MBTA, called the Green Line extension “one of the best projects” in the country and said that the new service will easily attract ridership. “This is not ‘build and they shall come,’ ” she said. “They are already here.”

Related coverage:

• Will the Green Line Extension make Somerville real estate go crazy?

• Green Line extension cost is going up several million dollars

• Will the Green Line Extension have a positive impact in Medford?

• Boston-area communities vie to be the next Somerville

Nicole Dungca can be reached at nicole.dungca@globe.com. Follow her on Twitter @ndungca.