The Massachusetts Bay Transportation Authority (MBTA) operates rapid transit (heavy rail), light rail, and bus rapid transit services in the Boston metropolitan area, collectively referred to as the rapid transit, subway, the T system, or simply the T.

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 Mattapan 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, and only 31 out of the system's 153 stations are located underground. The five branches of the Silver Line bus network are also shown as part of the rapid transit system. Three branches operate underground and charge rapid transit fares; two branches operate entirely on the surface and charge lower bus fares.

In 2025, the heavy rail lines had 101,054,600 rides, or about 343,800 per weekday in the fourth quarter of 2025, and comprised the fourth-busiest heavy rail system in the United States. In the same period, the light rail lines had 42,065,000 rides, or about 117,200 per weekday, and comprised the third-busiest light rail system in the United States.

The section of the Tremont Street subway between Park Street and Boylston Street stations, now on the Green Line, opened in 1897, making it the oldest transit subway in North America still in use (preceded only by the short-lived Beach Pneumatic Transit demonstration line in New York City).

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 continued in the 2000s with the addition of Silver Line bus rapid transit and Green Line Extension.^{[citation needed]} (See MBTA History and MBTA Future plans sections.)

Streetcar congestion in downtown Boston led to the creation of underground subways and elevated rail, the former in 1897 and the latter in 1901. The Tremont Street subway was the first rapid transit tunnel in the United States and had a 24/7 service. The grade-separated railways added transportation capacity while avoiding delays caused by intersections with cross streets. The first elevated railway and the first rapid transit line in Boston were built three years before the first underground line of the New York City Subway, but 34 years after the first London Underground lines, and 29 years after the first elevated railway in New York City.

Various extensions and branches were added to the subway lines at both ends, bypassing more surface tracks. As grade-separated lines were extended, street-running lines were cut back for faster downtown service. The last elevated heavy rail or "El" inter-station segments in Boston – with the exception of the Red Line's still-active elevated tracks, connecting Charles/MGH station over Charles Circle to the Longfellow Bridge and the Cambridge Tunnel's northern portal – were at the extremities of the Orange Line: its northern end was relocated in 1975 from Everett to Malden, MA, and its southern end was relocated into the Southwest Corridor in 1987. However, the Green Line's Causeway Street Elevated remained in service until 2004, when it was relocated into a tunnel with an incline to reconnect to the Lechmere Viaduct.

The final section of elevated, between the Lechmere Viaduct and Lechmere station, was closed in 2020 for construction of the Green Line Extension. The extension opened in two phases in 2022 with new branches from Lechmere to Union Square and Medford/​Tufts – the first rail expansion since 1987.