The color-branded lines consist of three heavy-rail lines (Red, Orange, and Blue), one branched light-rail system (Green), and a short light-rail line (the Ashmont–Mattapan High-Speed Line, colored as part of the Red Line). All except the Ashmont–Mattapan line operate in tunnels in the downtown area, but no route operates entirely underground. Only 26 out of the system's 133 stations are located underground. The five branches of the Silver Line bus are also shown as part of the rapid transit system. operate underground as bus rapid transit and charge rapid-transit fares; operate entirely on the surface and charge lower bus fares.