You're clearly not a 'station connection' game follower, Calthrop - the categories feature quite a bit there ( which is how I first heard of them about three weeks ago). I can't cut and paste links on my kindle, but googling United Kingdom railway station categories will show you a Wikipedia page explaining the whole shebang and enriching your life immeasurably. If, understandably, you can't be bothered, E and F are very small stations.