Interesting topic. I know I have to press the button in on my cam approximately 1 - 1.5 seconds before the shot I want is there. I always do a countdown for a moving train. Guess the distance, estimate the speed, work out from that how long it'll take to get the subject in frame, count down from there. Usually a 5-second countdown for me, at a moderate speed. If it's storming, it's a 3-second countdown, and counted at speed, with a bit of luck involved.
Good point about digital cameras, much, much easier to take a lot of shots and choose the best ones. It's the main reason (apart from dev costs) that I'd NEVER return to film. That is, unless I wanted some extremely good shots that I could take my time over, and get 100% perfect for each capture. Then I'd get an SLR out.
For a trip around London though, digital leads the way.