We use ON for all forms of public transport and of course getting on a horse, bike, motorbike etc. We use IN when it isn't public transport such as a car or taxi.
This is most notable when we get INTO a private plane we don't say ONTO.
I have a similar dilemma about where I live. Am I in the Isle of Man, or on the Isle of Man? I think I use 'on' when needing to make my whereabouts clear (eg, ordering stuff by phone).
Most veteran, Edwardian and Vintage cars were open cars and people in those eras did say "on a car." It was the rise in popularity of saloon car bodies that saw people begin to say "in" (for inside) the car.' If you read old motor racing books or listen to the commentary of films from the 'thirties, you'll find the drivers were always "on" such and such a car, "Bira on his Bugattii was to finish in second place" not "in."