To answer your question, we need to ask ourselves when a hobby becomes a fetish. Because honestly, when you get right down to it, that's the distinction we're making when we separate the buffs from the foamers.
In my opinion, it's all a matter of moderation. For example, I would feel totally comfortable if someone called me a buff. I like to visit other railroads and see their equipment, and if I happen to be near a museum, I make every effort to check it out. I mean, come on. Trains are totally sweet. But if you bumped into me at a museum, you'd notice a couple things about me. First, I'm not carrying a camera. My memory is good enough that I really don't need one. Second, I'm not wearing anything railroad related. No hats, no shirts, no pins, no nothing. And third (and most important), I'm not spouting off technical information or bogus stories that I've heard second-hand or read in magazines. I enjoy my visit, but I show a little reservation. Pretty much your basic buff.
On the other side of the line, we have our foamers. Foamers are the guys who foam from the mouth and the groin as soon a railroad touches their peripheral vision. They're the guys you see decked out in railroad apparal. They'll have two cameras with them, and a scanner rigged up to their car's stereo system. You see them crawling under rolling stock looking for truck castings, or hanging off overpasses taking pictures. They ask you questions to see if you know the answer. Know what I mean? Well, those are preliminary indicators, but to really be sure, you have to take some time and talk to them.
The first thing you'll notice is that a foamer is way over-eager. He wants you to know that he's an expert, and he'll really let you know it. Sometimes they'll go to the point of rudeness, correcting you on trivial details. Just a little condescending! They also try to share in the attitudes of railroad employees, but it comes off as a contrived effort. They'll make comments about how useless CMO's are, for example, or they'll slam railroad officials for not knowing how operations work on the ground. That bugs me a lot. They also have weird loyalties to certain railroads and locomotives, and they'll totally glamorize them. One guy said to me the other day, "Nothing beats a good smoking Alco!" Yeah? He's obviously never had to run one of those piles.
Well, I'm starting to rant. Foamers are alright, but in small doses. It's fun to play the cool engineer and let a couple dudes into the cab. Depending on who's around, we'll even let them ride a little while we switch. As long as they cap the camera lens, if you know what I'm sayin'. But yeah, anyway. Like I said, the difference between a buff and a foamer is all about moderation. Trains make a good hobby, but as far as fetishes, do your shopping somewhere else.