Boylston Street subway

In operation since 1914, it now carries all four branches of the MBTA Green Line from Kenmore Square under the Back Bay into downtown Boston, where it joins with the older Tremont Street subway.

The Huntington Avenue subway branches off to the south just to the west of Copley.

[1]: 44 The depth of the tunnel varies based on street considerations and other geography.

At Arlington, Hynes Convention Center, and Kenmore, the tunnel is substantially below street level, and the stations have below-ground fare mezzanines offering access to both platforms; at Copley, the tunnel is shallower, and the fare gates are at platform level, with a free crossover to reverse direction not possible.

The deepest section of the tunnel is under the Muddy River, in the Fenway neighborhood of Boston.