One of the best ways to explore a city is via mass transportation. It's an easy and inexpensive way to get around. Above-ground transportation like buses, trams and ferries offers close-up views of local neighborhoods. Subways, trains and light rail let you pass by the traffic jams that often plague the world's big cities, and get you where you're going quickly, leaving you more of that coveted leisure time.
The cities on this list were chosen for their extensive public transportation networks or because they have at least one state-of-the-art or especially convenient transportation mode. Each city on the list also offers special passes for tourists and "smart cards" (electronic cards you pass over a sensor for entry or exit) that include discounts for airport transportation, multiple rides and savings at popular destinations.
A word of caution: Tourists traveling on mass-transit systems, particularly late at night, can be targets for criminals. Do your research, plan and find out about safety issues and what to look out for. Familiarize yourself as much as you can with schedules, routes and transfer options before beginning your trip so the memories you bring home are only good ones.