The subway is the oldest, the most interesting and the quirkiest part of the T system. An integral part of downtown Boston, it runs on four different lines which have been color-coded for ease of use. Each line has inbound and outbound directions, which do not correspond with the compass directions and which change as you pass through the line's center point. Confusing matters further, not all lines connect at a central hub. The Red and Green lines connect at Park St., for example, while the Orange and Blue lines connect at State Street. The Blue and Red lines never meet.
The Green Line splits off into four branches in the westbound direction, so be sure you are on the right train when you are heading that way. The 'B' branch goes to Boston College via Commonwealth Avenue; the 'C' branch goes to Cleveland Circle via Beacon Street; the 'D' branch goes to Riverside where it meets Route 128 and the commuter rail; and the 'E' branch goes to Forest Hills via Huntington Avenue. Past the Kenmore Square stop, these trains run above ground, on a special right-of-way in the middle of the street. They stop every few blocks -- just ring the bell on the cord along the outside wall to signal the driver that you want to get off.
The subway has the most frequent service, with trains arriving every few minutes during rush hour. The subway also runs the most hours per day, from 5 AM to 12:30 AM.
Work on Boston's subway began in the 1890s, making it the oldest rapid transit system in the U.S. As you travel underground, you can occasionally glimpse abandoned tunnels leading away from the main line. See these from the safety of the car or platform; they are not safe places to explore!