Opened in September 1897, the four-track-wide segment of the Green Line tunnel between Park Street and Boylston stations was the first subway in the United States, and has been designated a National Historic Landmark. The downtown portions of what are now the Green, Orange, Blue, and Red line tunnels were all in service by 1912. Additions to the rapid transit network occurred in most decades of the 1900s, and continue in the 2000s with the addition of Silver Line bus rapid transit and planned Green Line expansion. (See MBTA History and MBTA Future plans sections.)