Prepositions

Should it be I'm on the bus or I'm in the bus? Go figure.

In Standard English, the prepositions in, into and out of are used to talk about travelling in, entering or leaving vehicles such as cars, taxis, ambulances, lorries, small boats or small planes (CCEU 2004:796). These are, generally speaking, smaller and lower vehicles.

1. When I travel in Cally's car, I grip the seat with fear.
2. Once Bobby gets in/into a taxi, he buries his face in a newspaper so he doesn't have to talk to the driver.
3. Jacob's knees were wobbly when he got out of the helicopter.

By contrast, the prepositions on, onto and off are used with other forms of transport, typically buses, coaches, trains, larger planes and ships - these are generally larger and higher vehicles.

1. He got onto the number 15 bus at Sixth Avenue.
2. It was an achievement for Roland to go four hours on the coach without smoking.
3. Selena got off the plane from Toronto and went straight to work.

The prepositions in, into and out of are, however, sometimes used with these forms of transport:

1. The drunk footballers were running around in the plane.
2. I couldn't wait to get back into the comfortably cool bus.
3. The terrified tourists leapt out of the burning coach.

Quoted from English Grammar FAQS by Leong & Tan (2008)