http://straylight.co.uk/?p=120 :
"As for the train (less cheap), I’d seen the service described as everything from high-speed to a bullet train, much to the amusement of anyone who’s spent time on British rail… On the other hand, my original post described how you could get a subjective speed of 5mph by 210fps filming at 35mph, so a lot of posters assumed that’s exactly what I’d done and that it was crawling along. Anyway: I was travelling on a Bristol-London Intercity, which I believe is capable of 125mph, but I’ve no idea how fast it was entering the station. Presumably there’s a limit even for non-stopping services, but obviously this one had to be slow enough not to overshoot the platform. People have tried estimating the speed based on everything from the likely length of benches and the time taken to pass them, to the flickering of lights on mains frequency, but it might be easier just to ask someone at the station next time I’m there. Update: Various people have claimed (without sources!) that the limit at Bath Spa is 35mph due to the curve of the platform."