For the UK or international. I am interested in studying how people's method of travel to/from a train station varies with station location characteristics.
In the UK, you may be able to get this through the National Travel Survey data, though I'm not sure if it's broken down on a station-by-station basis. It looks like it may simply be regional.
Within London, I believe TfL collect more data and do so at the station-specific level - I haven't been able to find out if they release the data, as their site is quite vague on what's available, but I know they collect it as I remember being given a questionnaire to fill out at Kings Cross a couple of years ago! This will be quite urban-focused, though, so probably not fully suitable for your needs.