I thought of something interesting this morning.
There are obviously a substantial number of amateur automobile drivers here in the U.S. I mean that not as a slight, but in the dictionary sense of the word - people who drive but are not paid to do so. In fact, amateurs probably substantially outnumber the professionals given that nearly everyone over the minimum age has a driver's license.
Although it's not quite as lopsided, this caries over to aviation as well. According to Wikipedia, at the end of 2011 there were just over 300,000 private and student pilots, and just over 250,000 commercial and airline transport pilots licensed in the U.S.
I don't have the figures handy for boating, but given the number of privately owned small craft, I can't imagine the number of amateur boaters isn't at least comparable to the number of professionals.
But then, there's trains.
There are only a handful of truly privately owned (that is, by an individual or a family rather than a business or a museum), full sized train locomotives, and an even smaller amount of privately owned (again, by an individual or family rather than by a railroad company) track. Almost nobody in the whole of North America is driving a train who isn't being paid to do so. Which also means that almost nobody in North America who knows how to drive a train learned how to do so other than through on-the-job training. Even Mike Rowe was being paid (albeit indirectly) to learn how to drive a train when he did it on Dirty Jobs.
I'd say that it's likely that in the U.S. there are more amateur submarine pilots than locomotive engineers, given that small recreational submersibles actually exist whereas there is no such thing as a recreational railroad (well, there are roller coasters, but those aren't actually driven by amateurs).
The closest you can possibly come to the insular cadre of professionals that drive trains is probably armored cavalry units of the military. Even then, I suspect that there are more armored vehicles and tanks in private hands in the U.S. than there are trains. And that's largely due to the transient nature of military service. Lots of folks spend their youth in the military and then move on to some other career. That's certainly when contrasted with railroad employees, who are more likely to keep that career for life.