A friend of mine recently posted a comment on Facebook saying that the phrase "on a train" is a pet peeve of his since it very rarely refers to somebody literally riding on top of a train.
This got me thinking about the prepositions used for other forms of transport and I noticed a pattern where "on" seems to be used for all forms of public transport even when it sounds like it should be technically incorrect (bus, plane) but only for private transport where it is technically correct ('on' for bike but 'in' for car).
Curious to see if there were any answer out there as to why this is I found a previous question that specifically related to trains "on the train" or "in the train"?. Most of the answers related to what the correct preposition is rather than why but one of the top rated answers suggested that on is used to distinguish between a stated of having boarded a train from being a component of a train. I'm not entirely sure I agree with this answer but it also doesn't really apply to other forms of public transport.
My best guess is that the use of 'on' is a remnant from a time when it would have been correct for most forms of mass transport (cart, boat) but I'd be interested to know if there is any other reason.