bit like trancekid said - trainspotting is picking the tunes, or trying to, while the dj is spinning. its not so much to do with watching them spin, its to do with
what they spin.
trainspotters/anoraks/anally-retentive idiots/whatever you want to call them, generally stand around the dj booth, just watching. not dancing, often not even moving - really looks like they're not enjoying it. they're noting what gets played, and that seems to consume them. the worst of them actually take notes - i kid you not. i've heard stories told by big name djs, where they were surrounded by a bunch of moody looking bastards, and some even taking notes on paper of whats being played. its a bizarre thing to do, but then real trainspotting is pretty bizarre as well
i think, at its worst, its contained to certain genres. i don't think you'd get the stereotypical 'spotter during a cheese-trance set, while if someone like the aphex twin was playing, you'd get the
real 'spotters in effect. for most more dance-floor oriented genres, they exist more like me and others in this thread, just watching a dj work, trying to firgure a tune or two out, every now and then. real trainspotters...yes, freaks