The advice that's been provided in this thread is solid -- but I need to emphasize something that will set you apart, when and if, you get that initial opportunity to learn an operating craft -- it's the way you approach the culture of safety. It makes no difference if you work on a so-called "real" railroad, or on a tourist train, unsafe practices can kill. Your supervisor or foreman will pay close attention to matters like your alertness, your adherence to and knowledge of the rulebook, the way you conduct yourself around equipment and on railroad property, and the way you relate to the actions of other workers. It also goes without saying that you must be able to pee in a bottle without negative consequences!
If I have a quibble with "foamers", it's that they sometimes let their passion for their hobby get in the way of common sense. When you're working, you've got to give full attention to the job at hand and not be distracted. (and before I get comments...yes, I DO know that many serious railfans pay as much attention to safety as "professional" railroaders do.)
Good Luck. Should you get a chance to work in this business,and I hope you do, don't forget the old adage... "The railroad rulebook is written in blood". Safety really IS a first obligation of railroad operating people.