The Wikipedia entry for railfan contains a small trove of unfamiliar-to-me words.

The term itself has the meaning you'd probably expect: someone who's interested in railroads. But the article also defines such terms as "FRN" (Fucking Rail Nut); "grizzers," "gricers," and "gunzels" (all terms for railfans); "bashers" (who like actually riding the trains); "stoats" and "veg," who spend their time mainly at one station; and more.

See also the articles on US railfan jargon and UK railfan jargon.