Use on for large vehicles which you can stand and walk around in (a bus, an airplane, a train, a metro/subway car, a cruise ship, a boat). Use in for (usually) smaller vehicles or crafts that you have to enter and sit in (a car, a taxi, a truck, a helicopter, a canoe, a kayak, a small boat, a carriage, a rickshaw).

The distinction between "in" a taxi and "on" a bus in the English language is a linguistic quirk based on the ability to stand and move around. Generally, we use the preposition "on" for modes of transportation that are large enough to have a "deck" or floor that you can walk across, such as a bus, a train, a plane, or a ship. You "board" these vehicles and stand on them before finding a seat. In contrast, we use "in" for smaller, private vehicles like a car or a taxi where you must crouch or stoop to enter and immediately sit down. You are "enclosed" within the small space rather than being on a platform. This rule also applies to bikes and horses ("on") versus enclosed cockpits like a fighter jet ("in"). While it may seem arbitrary to non-native speakers, it is a consistent rule of thumb: if the vehicle has an aisle you can walk down, you are usually "on" it; if you just sit in a seat and pull the door shut around you, you are "in" it.