Wow, having just read through this thread it would appear that there's a lot of barely suppressed rage on CC.
Anyway my personal pet hate is people who insist on speaking on their mobile phones in the quiet of train even though they are perfectly aware that it is a quiet coach. They usually start their calls by loudly proclaiming "I mustn't be long because I'm in the quiet coach", and then proceed to talk volubly and at length often ignoring appeals from their fellow passengers to be quiet.