In 1897, the Boston Subway became the first subway in the Unites States when a segment of Green Line tunnel opened from Park Street Station to Boylston Street. By 1912, the current Green, Orange, Blue, and Red lines were all in service downtown. Since opening, additions and improvements have continually been made, including construction of the Silver Line and expansion of the Green Line. Today, subway riders account for about 55 per cent of the Massachusetts Bay Transportation Authority (MBTA) system users.
The system operates according to a spoke-hub distribution paradigm, with lines running between central Boston and its surrounding areas. All four colored lines meet downtown at a square configuration. The subway system consists of three rapid transit lines—the Red, Orange and Blue Lines, and two light rail lines—the Green Line and the Ashmont–Mattapan High Speed Line (which is part of the Red Line). The colors were assigned to the lines on August 26, 1965 in conjunction with design standards developed by Cambridge Seven Associates. The colors have served as the primary identifier for the lines since the reorganization of the Metropolitan Transit Authority (MTA) into the MBTA.
There are no direct track connections between lines, except between the Red Line and Ashmont-Mattapan High Speed Line. All lines except the Blue Line have currently unused connections to the national rail network, which have been used for deliveries. Because the three rapid transit lines are incompatible, trains of one line would have to be modified to run on another. Orange and Blue Line trains are similar enough that modification of some Blue Line trains for operation on the Orange Line has been considered, although ultimately rejected for cost reasons.
The Orange and Green Lines run parallel and meet directly at two stations. The Red Line has two branches in the south named after their terminal stations, Ashmont and Braintree. The section from Harvard to Park Street Under represented the city's first rapid transit subway, 1912. The Green Line has four branches in the west—"B" (Boston College), "C" (Cleveland Circle), "D" (Riverside) and "E" (Heath Street).
In the 2009 Report Card for America's Infrastructure, transit systems received a poor grade -- D, with a projected spending shortfall of $190.1 billion over the next five years. To learn more about the state of transit in the U.S., read the Report Card.