A potential Red Line-Blue Line connector is back in the spotlight

Every now and then, it seems, one of the long-debated MBTA expansion projects resurfaces and captures the public’s attention for a brief period before quickly receding into the annals of ideas gone by.

That has been the lifecycle of the Red-Blue connector — the on-again, off-again idea to extend the Blue Line a short distance down Cambridge Street to the Charles/MGH Red Line Station, finally linking the only two subway lines that don’t currently connect.

Now, as Massachusetts General Hospital prepares a massive expansion of its nearby campus, the Red-Blue connector is getting another turn in the spotlight. But this time, the Massachusetts Bay Transportation Authority seems serious enough about the idea to have held about a dozen meetings with MGH, poring over detailed engineering plans and snazzy renderings as they consider how the two projects would coexist.

As part of its $1.9 billion expansion, MGH last month unveiled a proposal to set aside space for an entrance to a new underground Blue Line station in one of two towers it wants to build along Cambridge Street. Plans seem far along, containing such details as where elevators and escalators would be located, how to separate transit and hospital facilities, and where to build an underground wall that will later get knocked down to provide access to the future Blue Line station.

“There’s more than just the agreement on the concept of doing it,” said Nicholas Haney, project manager for planning and construction at MGH. “There are a lot of details we had to take into consideration. ... We didn’t want to say yes to something, and then not be able to build it in the future. We had to figure out a lot of it out now.”

To be sure, this commitment from MGH does not ensure the Blue Line extension will happen. The hospital’s plans are limited to building an area to access the Blue Line station — not the station itself, or the tunneling and rail construction under Cambridge Street. In some ways, the conversations seem less focused on facilitating the Red-Blue connector, and more about ensuring the hospital expansion won’t block the transit project in the future.

“Over the past year, the MBTA has advanced a feasibility review for the Red-Blue Connector project to ensure that future development in the corridor can advance without compromising the potential future extension of the Blue Line to Charles/MGH,” MBTA spokesman Joe Pesaturo said.

For its part, MGH expects to complete its expansion by 2030; meanwhile, the Red-Blue connector has no clear timeline. The space set aside for the station entrance, hospital officials said, may wind up getting leased to a retailer until the MBTA is ready.

Still, supporters of the connector are encouraged by the level of detail of the plans, and the attention the MBTA and MGH are giving to an idea that was once left for dead.

“The meetings are a good sign,” said James Aloisi, a former state transportation secretary who has long advocated for the connection. “I don’t know if it’s anything definitive, but I would read into it ... that people are more congenial than they were five years ago.”

The connector was promised years ago as environmental mitigation for the Big Dig, similar to the long-awaited Green Line extension that is finally wrapping up this year. But it was set aside by the administration of former governor Deval Patrick, and the federal government allowed the state to escape the commitment in 2015, after Governor Charlie Baker had taken office.

It returned to the forefront in 2017, however, when Boston officials tried to court Amazon to open a headquarters at Suffolk Downs. At the time, the city described the expansion as a “clear goal” of the state — even though the state and federal governments had just formally shelved the project.

Still, that was enough of a spark to get state transit officials to take another look at the costs that would be associated with the proposal, once estimated at $750 million.

The T later determined that, depending on the construction method, it could build a tunnel for much less, perhaps as low as $200 million to $350 million. (The MBTA has stressed that this does not include work beyond the tunneling, such as tracks, power systems, platforms, and station infrastructure, which would add considerably to the total.)

Even so, there still is no clear funding source for the project. The MBTA has already put about $2 million into planning, expects to spend more than $3.5 million to advance the project next year, and will have about $9.5 million available beyond that. But that is unlikely to cover even the costs of permitting and engineering, Pesaturo said, never mind construction.

A view of Cambridge Street, near the Charles/MGH T Station with the tracks overhead. The long-proposed connector would extend the Blue Line from Bowdoin to Charles/MGH by tunneling under Cambridge Street. David L. Ryan/Globe Staff

The state is authorized to borrow money to fund the project, though it would come out of a much broader pool of funds for the entire transit system. Some supporters also hope that a potential federal infrastructure bill could help the MBTA pay for the connector.

Despite the financial uncertainty, state Senator Joseph Boncore, a Winthrop Democrat who leads the Senate’s transportation initiatives, said the connector should be expedited so it can be completed in tandem with the MGH expansion, since the area will already be undergoing significant construction.

“It’s an opportunity to marry the projects,” he said. “We need to seize the opportunity.”

In other parts of the region, developers have increasingly helped fund nearby transit projects, including subsidies of more than $10 million for the Assembly Row Orange Line Station and the Boston Landing commuter rail stop.

MGH, however, seems uninterested in ponying up much more money. The hospital notes that it was asked to include the station accessway as part of the project, and that alone will likely cost $5 million or more.

And although MGH’s employees and patients would benefit from the transit connection, senior vice president of administration and finance Sally Mason Boemer noted that the hospital will lose out on potential rental income “in perpetuity” by setting aside the space for the MBTA.

The project’s most prominent internal supporter at the MBTA is Joseph Aiello, the chairman of the agency’s governing board — though that board is scheduled to disband at the end of June. He describes the connector as crucial to East Boston, Revere, and Chelsea, providing an opportunity to link immigrant communities along the Blue Line with jobs in Kendall Square and elsewhere on the Red Line. It would also provide a degree of “resilience” for the system if trains break down downtown, making it easier for passengers to make other connections, he said.

But as is so often the case with these big-ticket projects, the absence of funding may pose the biggest challenge for a proposal that is competing with other major MBTA plans, such as buying new Green Line cars and electrifying the bus fleet.

“This falls in line with a whole series of other initiatives that are important to the future of the community,” Aiello said. “The struggle will be trying to figure out the sequencing of these things, and the dollars to make these things happen.”