THE MBTA unveiled an $850 million cost estimate for connecting the Red and Blue subway lines at Charles/MGH, a project that, if funding is found, could break ground in 2025 and be completed by 2030.

In a presentation to the Fiscal and Management Control Board on Monday, Eric Stoothoff, the T’s chief engineer, described how the connector would be built and a “concept” for what it would look like. He said Bowdoin Station on the Blue Line would be shut down and the Blue Line extended underground down Cambridge Street for 2,200 feet to a new Blue Line station at Charles/MGH.

Stoothoff said the connector would be built using a so-called cut and cover construction method – a huge trench would be dug down Cambridge Street, the subway line would be built in place, and then the trench would be recovered. Passengers would exit the new Blue Line station via escalators to the lobby of the existing Charles/MGH station or to a proposed Mass General Hospital medical building on Cambridge Street.

A map showing the current Blue Line configuration. Under current plans, Bowdoin Station would be closed and the Blue Line would bar extended 2,200 feet to the Charles/MGH Red Line station between Cambridge Street.

The idea of a Blue-Red connector has been bandied about since the early 1970s. State officials committed to a connector in 1990 as mitigation for the Big Dig, but that promise was abandoned due to lack of funding in 2010 and the federal government gave its blessing to halting the project in 2014.

The idea has gained momentum in recent years as a way to give residents north of Boston – particularly those in Chelsea, Revere, and East Boston – a way to connect more easily with the Red Line and get to Massachusetts General Hospital.  The idea also offers better connections to the airport for Red Line riders coming from Cambridge and Somerville.

Revere Mayor Brian Arrigo and Chelsea City Manager Tom Ambrosino both testified in favor of moving ahead with the project on Monday, as did Boston mayoral candidate Michelle Wu, former state transportatiuon secretary James Aloisi, and a host of advocates.

The spending plan for the concept estimates the cost of tunnel construction at $270 million, or nearly $114,000 per foot. The  cost of a new Blue Line station was estimated at $60 million and the reconstruction of Cambridge Street at $20 million. The spending plan includes $140 million for contingencies, $50 million for additional design work, $130 million for inflation, and $130 million for other measures.

The T has spent limited funds on some preliminary designs and has $15 million on hand to do more design work. But that leaves most of the cost unfunded. Joe Aiello, the chair of the control board and a leading proponent of the connector, said he assumed acting Transportation Secretary Jamie Tesler and MBTA General Manager Steve Poftak would include funds in future capital spending plans for the project, but no commitments were made at Monday’s meeting.

A separate presentation to the control board suggested the T will face a huge gap in the future between potential capital projects and available funding. One chart showed funding needs in the $3 billion range for each of the next nine years with available funding nowhere near that level. The gap starts growing particularly large in fiscal 2024 and rises to about $2.5 billion a year in fiscal 2026 and beyond.