Quora is one of the best social networks in terms of questions and answers. Needless to say, there are numerous questions and answers about the English language. This is a particularly good example:

QUESTION

Why do we say “on the bus” or “on the train” but “in the car”?

ANSWER

For vehicles, anything that has a large floor space is ON and a small floor space is IN.
Large floor spaces are considered platforms, small floors are considered containers. Therefore it’s in a car, in a small boat, in a helicopter, in a rocket, in a small plane. But we say on a train, on a ship, on the space shuttle, etc. When the platform has no walls we say on, too. So it’s on a stage, on a table, on a horse, on a bike, on a skateboard. Because the language is dynamic, and because it is old, there are some exceptions but this is the basic rule. Once you begin to visualize objects as platforms or surfaces, on becomes logical too for other kinds of objects: on a screen, on paper, on a page and so on.

Chris Aubeck on Quora