I read in the book that John had been a train engineer for six months, maybe 10, 15 years ago.--Also he had done some mining in Ireland. ??
I think the train engineer thing was added in the book simply to explain why in the movie Sean knows how to operate a steam locomotive which is extremely difficult and almost impossible to do without studying it's controls and how they work.
Most likely Sean, as many people believe, was rich or had connections to the upper class. Remember that cars and motorcycles in 1910 were extremely expensive. The automobile wasn't mass produced until Ford's assembly line models in the 20's when the prices dropped low enough for the average American.