Originally Posted by
lairroad
I think if you booked the "special" coach fare and tried to upgrade on board, you could. If you tried to book the special fare and then combine it with a sleeper days in advance, I could see how that would make it ineligable. If there are open sleepers, they try to sell them and sometimes even make announcements on certain trains.
You are correct, one cannot upgrade rail sale ticket pre-departure, but once on board if a room is available then one can upgrade for the lowest bucket price for that room/distance.