If you can stand up or walk inside the vehicle, you should use the preposition 'ON'. On the other hand, if you can't stand up or walk inside the vehicle and if you can only sit in the vehicle, you should use the preposition 'IN'.
A) Can Stand Up or Walk Inside [Use 'ON']
When we travel by train or by bus, we can stand up and walk inside the train or bus. In this case, you should use 'ON'.
For Example,
1) I'm On the train.
2) I'm On the bus.
3) You're On the ship/plane.
* [On a plane/a ship/a bus/a train/a bicycle]
B) Can't Stand Up or Walk Inside (Only Can Sit Inside) [Use 'IN']
When you travel by any private vehicle or any other vehicle in which you can't stand up or walk inside it, you should use the preposition 'IN'.
For Example,
1) I'm In a car.
2) John is In the car.
3) Tom is In a taxi/helicopter.
* [In a car/a helicopter/a truck/a taxi]
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