Genuine question.
I used to think that it was one coach of the train reserved for a quiet atmosphere. However, this quite clearly isn't true as half the time there's someone having a loud banal phone conversation, or narrating their entire day out loud, or listening to the latest radio one shite.
Being British, most people (myself included
) usually just give the death stare and a few tuts, but not actually challenge the culprit directly...