Together, the steps amounted to a bid to help salvage most of the 4.7-mile rail line, which has run up to $1 billion over budget. State transportation officials have threatened to pull the plug if that gap can't be closed. The offers would still need to be approved by the two city councils.

The offer from the cities of Somerville and Cambridge came as a regional transportation planning agency also took steps to shift $152 million that would have extended the rail line to Route 16 in Medford and instead spend that money on an earlier phase of the project that runs through Somerville.

In a last-ditch effort to save the troubled Green Line extension, leaders of two cities on the rail line said Thursday they'd contribute $75 million toward the project — just four days before transportation officials will meet on its fate.


"It became very clear to us that to have any shot of moving the Green Line forward, we'd have to make a significant contribution," said Somerville Mayor Joe Curtatone, whose city is offering $50 million. "We didn't come to this decision lightly." Six of the seven planned Green Line stations would be located in Somerville.

On Monday consultants hired by the Massachusetts Department of Transportation are set to present redesigned, scaled-down stations, contracting changes, and other plans aimed at reducing the cost of the now-$3 billion project. The boards of MassDOT and the Massachusetts Bay Transportation Authority's fiscal control board could then vote to approve the revised plan — or to kill the project altogether — perhaps as soon as Monday.

Members of the fiscal control board — appointed last year by Governor Charlie Baker to help right the MBTA's finances — did not return calls late Thursday. Transportation Secretary Stephanie Pollack praised the new contributions, which were hashed out over months of talks with city and state officials, but said little about the plan to be unveiled Monday.


"MassDOT appreciates the collaboration by Cambridge, Somerville, and the Boston Region Metropolitan Planning Organization throughout this process and looks forward to further discussion of the redesign and its cost with the boards next week," she said in a statement.

Advocates for the line — who have packed public meetings in recent weeks urging officials to find a solution that gets the project built — cheered the news, even as they acknowledged $75 million is a relative drop in the bucket.

"It's a great vote of confidence, obviously. It puts more skin in the game for Somerville and Cambridge," said Marc Ebuña, cofounder of Transit Matters, which backs the extension. But "it's more of a gesture than it is necessarily going to [move] the project forward."

Others framed the move differently. State Representative Denise Provost, a Somerville Democrat and vocal supporter of the extension, said her understanding was that state officials gave the cities no choice but to pony up.

"The Commonwealth has the power to hold the Green Line extension hostage," she said. "You could view this as asking for ransom."

Either way, coupled with the $152 million that had been earmarked for the project's final phase and could now be spent earlier, the money should be enough to make a meaningful difference, said Marc Draisen, executive director of the Metropolitan Area Planning Council.

"That's a significant chunk of change," said Draisen, who's also vice chairman of the MPO, which allocates federal transportation funding in the region. He said he was hopeful that the $152 million could eventually be replaced to fund the line all the way to Route 16. "We don't have the final cost numbers yet, but I'm hoping the gap has been very, very substantially narrowed."


The move is substantial in another way, Draisen notes. It's a rare case in Massachusetts where local governments would help finance transportation projects — typically the domain of the state. With funding tight for both transit and road projects statewide, he said, it could be a way to help build more.

And in this case Cambridge has another partner. The private developers who are building NorthPoint — a 42-acre complex of office, retail, and apartment buildings in East Cambridge that would house a new Lechmere Station — are in talks to foot some of Cambridge's $25 million bill.

"We will continue to work closely with them toward the completion of this essential infrastructure investment," said Tom Sullivan, president of development for NorthPoint.

And both state and city officials said they'll continue to explore so-called "value capture" tools that would help governments raise money from new development along the line to help finance its construction. But those tools require state legislation that is moving much more slowly than MassDOT's review of the overall project.

Construction of the extension, planned for 25 years, was originally intended to be complete in 2011 — and then in 2015. Some work on the project has begun, but the new cost estimates threw everything into doubt.


The offers from the two cities and the planning group are not set in stone.

Both Curtatone and Cambridge City Manager Richard Rossi said they're simply recommending the spending to their respective City Councils; any funds would need a vote. They're still working out details such as timing and where, exactly, in their city budgets the money would come from. And the new plans and decisions to shift federal money would need more local votes and the OK of federal officials.

But the top officials of the two cities that would benefit most from the Green Line Extension said they wanted to send a clear message — ahead of Monday's meeting — that they're willing to do what they can to help the project survive.

"There's a lot at stake for us," Rossi said. "It'll be quite devastating if this project goes down."

Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter at @bytimlogan.