Boats can be used as public transit, too. In this picture, a ferry boat takes commuters to San Francisco. Customers can buy sandwiches and drinks, and eat them while they stand on the deck and look at the bay. Most of the time, though, public transit means traveling on land. One very popular form of public transit is the bus.
Buses are much cheaper than trains, and seat fewer people.
They're best at serving transit routes that aren't heavily used. Some planners think that buses are almost always the best form of transit, because of the savings in money. The city of Ottawa, Canada built a famous transit system using buses. Special busways run through the city. These are wide roads like freeways, but only buses can use them.