I think i've only ever mentioned my railway interests to 2 friends, simply because they asked. I've never openly admitted having an interest in railways, simply because a lot of people I know/knew would (have) made fun of me. The people I've told, it was just in the passing, and I would tell more people if it came into conversation. Some people, though, I'd never tell. Yes, it's good to be open about things, but these are people who i know would just ridicule me publically for life, and I can't be bothered with that.
At work, on our first day, we were given the usual "getting to know you" rubbish where you stand up in front of your colleagues and tell them about your life. I was first and didn't mention railways just in case it was like school (which it is) and I'm glad. A few of my colleagues had interests, but many of them didn't seem to have any. Interests included dancing at a place called "megabar" :?, getting your hair cut, "just hangin' out", driving about... I can't see how any of them are interests at all.