[mykwyner]

Here's what I think about the usage of these prepositions: People (and things) ride in private transportation and on mass transit. There's no rule about it in English; it's all idiomatic.

This morning I got on a bus, rode it to the train station, got on a train, went to the airport and got on a plane. I might ride in a taxicab, in a car or in a motorhome (caravan). As for trucks, if I'm in the driver's compartment, I'm in the truck; everything in the cargo area is on the truck.