things/vehicles that you physically get onto are always on.
Examples: motorcycle, pushbike(bicycle), horse. These we always get "on" because we cant get "inside" them.
Other things like trains, buses and public transport are either in or on, but more commonly on.
Examples: Riding on the subway, travelling on the bus.
Private transport, such as cars, are always in.
Examples: Lets go for a ride in the car. Lets go for a drive in the car. Can you take me to the shop in the car please?