Boston - Boston University - Green Line
The "B" Branch of the Green Line, also called the Commonwealth Avenue Branch or Boston College Branch, on the Massachusetts Bay Transportation Authority (MBTA, or T for short) runs on a surface right-of-way down the middle of Commonwealth Avenue. After going underground via the Blandford Street Incline, the tracks merge with the "C" and "D" Branches into Kenmore station. From there the Boylston Street Subway and Tremont Street Subway carry "B" cars to downtown Boston, with regular service turning around at Government Center. The "E" Branch splits to the southwest just west of Copley into the Huntington Avenue Subway. From 1942 to 1967, this route was known by the map number of 62. Since then it has been the "B" Branch.
The Green Line, a light rail/streetcar system, is the oldest line of Boston's subway and the most heavily-used light rail line in the country. Given the green color because it runs though the area called the Emerald Necklace, the four branches are the remnants of a once large system, begun in 1856 with the Cambridge Horse Railroad. The Tremont Street Subway carries cars of all branches under downtown, and is the oldest subway tunnel in North America, opened in stages between September 1, 1897 and September 3, 1898 to take streetcars off surface streets.
In 1896, tracks were laid on Commonwealth Avenue from Chestnut Hill Avenue west to the Newton town line at Lake Street, and around the same time the Commonwealth Avenue Street Railway opened, extending the tracks through Newton to Norumbega Park; this later became part of the Middlesex and Boston Street Railway. Trains between Lake Street and downtown Boston used tracks on Beacon Street, now part of the "C" Branch. From Kenmore Square they continued east on Beacon Street, then turned south on Massachusetts Avenue and east on Boylston Street to Park Square. In 1900 tracks were installed on the rest of Commonwealth Avenue, from Chestnut Hill Avenue east to existing tracks at Packard's Corner, later part of the "A" Branch. In 1909, the tracks were electrified.
After the Tremont Street Subway opened, the Commonwealth Avenue line was rerouted to turn around at Park Street via the Boylston Street Incline at the Public Gardens. The Boylston Street Subway opened on October 3, 1914, extending the underground portion to the Kenmore Incline just east of Kenmore Square. On October 23, 1932 the Blandford Street Incline opened along with the underground Kenmore station, giving the line its present configuration.