The problems of photography on railway stations due to unknowledgeable staff is a frequent topic of discussion on the RailUK forums website. The majority of complaints seem to indicate that security staff in particular, hired in from non-railway based companies, are often the worst offenders in this respect. By their nature they are often unwilling to back down from their "Thou shalt not take photos here." attitude even when presented with the document John Hinson mentions! However, I notice the number of complaints in the last year or two seems to have dropped off, so perhaps the message that photography is allowed is getting through.
I've done little photography on the railways in recent years except on preserved lines, although when photographing St Pancras a year or two after its refurbishment I had absolutely no problems whatsoever.
(Member, St Albans Signalbox Trust)