The Tremont Street subway in Boston's MBTA subway system is the oldest subway tunnel in North America and the third-oldest still in use worldwide to exclusively use electric traction, opening on September 1, 1897. It was originally built, under the supervision of Howard A. Carson as chief engineer, to get streetcar lines off the traffic-clogged streets, instead of as a true rapid transit line. It now forms the central part of the Green Line, connecting Boylston Street to Park Street and Government Center stations.

The Green Line is a semi-metro system run by the Massachusetts Bay Transportation Authority (MBTA) in the Boston, Massachusetts, metropolitan area. It is the oldest MBTA subway line, and with tunnel sections dating from 1897, the oldest subway in North America. It runs underground through downtown Boston, and on the surface into inner suburbs via six branches on radial boulevards and grade-separated alignments. With an average daily weekday ridership of 137,700 in 2019, it is the third-most heavily used light rail system in the country. The line was assigned the green color in 1967 during a systemwide rebranding because several branches pass through sections of the Emerald Necklace of Boston.