Agreed. I have never had a problem with re riding at Thorpe, and like you said, even if the train is full you can just go behind the air gates again.
At Towers it was very annoying, they were using 1/2 trains in the station on Oblivion, most of the seats were empty and they still wouldn't let us re ride, we had to walk all the way around the queue every single time, very annoying.
With Chessington I think they should either allow re rides, like thorpe or not allow them at all like towers. It could be very confusing for quests and staff, having to count and will result in delays like mentioned on fury.