Boston's subway — the MBTA, universally called “the T” — is the easiest way to move between neighbourhoods, and it is especially welcome after a few rums. Five colour-coded lines cover everywhere you are likely to go, and the Red Line, linking Cambridge to downtown Boston, will do most of the work.
A few places worth the trip sit beyond the subway. The Commuter Rail reaches those — the Fitchburg Line out to Waltham, for instance — and runs less often than the subway, so check the timetable before you set out, and check the last train back.
You do not need to buy anything in advance: tap a contactless card or phone straight at the gate. If you would rather have a pass, CharlieCards and multi-day LinkPasses are sold from the fare vending machines in major stations. Current prices are at mbta.com/fares.
Service runs from roughly 5:00 AM to somewhere between 12:30 and 1:00 AM. Not every station is step-free — check mbta.com/accessibility before you plan a route.