A notional design for a Charles-MGH station on the MBTA Blue Line presented to T officials on June 7, 2021. Image courtesy of the MBTA

A rail extension linking the MBTA’s Red and Blue Lines could cost $850 million and require a design and construction period of nearly a decade, according to the latest estimates unveiled Monday.

In a presentation to the transit agency’s board, MBTA Chief Engineer Erik Stoothoff outlined an updated scope for the project that would feature a more than 2,000-foot tunnel underneath Cambridge Street to lengthen the Blue Line from its current endpoint at Bowdoin Station to the Charles/MGH Red Line station.

The preliminary plan would involve shuttering Bowdoin Station permanently, adding Blue Line platforms at Charles/MGH, and connecting the station to a clinical building to be constructed at Massachusetts General Hospital, Stoothoff said.

The presentation estimated total construction costs at $740 million – including $140 million for contingencies and $130 million for cost escalations – and $110 million in design, administration and train costs.

While project funding sources have not been secured yet, Stoothoff said the T could start procurement this year, spend about four years on the design, then run construction from late 2025 until completion around 2030.

North Shore elected officials and transportation advocates have long pushed to connect the Red and Blue lines – the only two MBTA subway lines that do not meet. The connection would allow Blue Line commuters from East Boston and Revere to reach job centers in Cambridge more easily, they say, helping to reduce congestion on highways and in tunnels, and relieve pressure on Orange and Green line trains, which riders on the Blue and Red lines must use to transfer.

Fiscal and Management Control Board Chair Joseph Aiello, observing that “traffic is coming back,” said after Stoothoff’s presentation that “this project can’t come soon enough.”