Boston is the capital of Massachusetts; with 620,000 inhabitants in the city proper, it is the centre of a large metropolitan area with some 4 million inhabitants.

The Boston Subway network (the 'T') consists of three colour-coded full metro lines (red, orange and blue), plus a light rail line (green line) with four western branches and an underground section in Central Boston (actually America's oldest subway opened in 1897). The three metro lines are underground in the city centre but elevated or at grade in outer areas, partly using former railway corridors. Like the Green Line, the Red Line crosses the Charles River on a bridge (Longfellow Bridge) towards Cambridge (Harvard University), whereas the Blue line dives under the Boston Harbor to the Airport station (from where a shuttle bus links all the terminals) and further east to Wonderland. From the Red Line's southern terminus at Ashmont, a tram line continues to Mattapan. The subway is operated by MBTA (Massachusetts Bay Transit Authority) and has a total length of 101.5 km.