All of the MBTA Green Line’s downtown portion and much of its branches will be closed for 15 days in December to fix a nearly 130-year-old tunnel piece, the transit agency announced.

From Dec. 8 through 22, the Green Line’s main branch will be closed from Kenmore to North Station. The E Branch will be entirely closed, and the B Branch will run from Boston College to Babcock Street, with shuttle buses running from Babcock to Back Bay. The C and D Branches will run only between their terminuses and Kenmore.

“For a planned shutdown, this is huge,” Brian Kane, the executive director of the MBTA Advisory Board, told MassLive . “That’s the entire trunk [of the Green Line], basically. That’s major.”

During the shutdown, the MBTA will replace the wooden trough inside the Green Line tunnel t

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