Why is it when you’re trying to describe to someone while you are utilizing public transit (bus, train, etc.) you say you’re “on the bus” or “on the train”. You aren’t technically on the vehicle, rather, you’re inside the vehicle. However, saying you’re in the bus or in the train leaves the impression that you’re somehow in the inner workings of the vehicle. Additionally, you don’t say you’re “on the car” rather you’re “in the car”.
Once upon a time, in England, it used to be that we had to climb on board a bus, train or ship. I guess that board means something like a platform. So, we say we're (travelling) on a bus, train or ship.
You cannot go on board a car, but we still say "all aboard!" especially for ships, trains and buses. And we need a boarding pass when we travel by air. Over time the word "board" was dropped, and now we're left with "on".