Boylston Street Subway

The Boylston Street subway is a light rail tunnel which lies primarily under Boylston Street in Boston, Massachusetts. 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 tunnel originally ended just east of Kenmore Square; it was extended under the square to new portals at Blandford Street and Saint Mary's Street in 1932.

Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).