I think they are born (or manufactured, depending on your viewpoint) with certain talents, like people, and decide later what they want to do “when they grow up” , we know cars can switch “jobs” like Doc was a race car then a doctor, so maybe busses are like that, they can choose what “bus job” they want (we all know that in our world there are multiple “bus jobs”)
What woutrain do? Their stuck on the tracks and boats are stuck on the water.
true, but they wouldn’t have to be a passenger train/boat, trains could transport all sorts of things, plus help the military (if needed) and boats could to anything on the water (depending only on size): coast guard (they could rescue sinking/drowning cars), Navy, construction, fire-fighting, ice-breaking (so the Eskimo-cars can get their much-needed supplies ~coughNome,Alaskacough~) transport of almost everything, as we saw in Cars 2, Crabbing.
In my eyes, perhaps a Fiat Multipla, or a Honda Insight 
I’m curious to see how horse led carriages/vehicles fit into the Cars universe.
Ditto 