On the train, on the bus, and on the plane, unless you're Bill Gates, Donald Trump, or me-I'm not in that list-you don't have your own train, plane, or bus, so you usually share it with a bunch of people or a few people. It's large. So we say: "You're on the bus", because it covers a big area, so there are many people sitting in that area. When I get to location, you'll see what I mean.