I think what Mike was suggesting about keeping your railfan status under your hat was meant more for the common carriers, who think foamers are the worst kind of employees. On tourist railroads, I think the better caveat is, leave railfanning at home when you work on the big stuff. At the MidContinent Rwy Museum here in WI, we have a formal training and advancement program. We have brakeman's classes each spring and the return of the sun always sprouts a new crop of dwarves--uh, students. It doesn't take long to size them up. The railfans that think this is playing trains spend too much time fanning and less time paying attention to learning their craft. The worst are the ones who stand there watching you go by with the engine on a run-around instead of getting the switch you need to make your move or not getting on the engine to get to the other end of the siding and make the joint.
But defintely try it. Preferrably somewhere with a formal training program. Expect that the work is hard, hot, dirty and sometimes down right unpleasant. And remember that everything weighs three times what you think it does and the tool needed to make a repair often weighs more than the part needing repair.
I regret when I looked for summer employment during college years that I wasn't smart enough to try tourist roads. I later learned that finding a career that gave me the money and freedom to get to a steam operation (they are never close by) was the better choice than railroading for a living.