They do it in Reading just before the festival.
So the platforms are overwhelmed with people queing for tickets.
The trains are packed.
Then as you didn’t leave an hour extra for the 10 minute journey to get a ticket you get fined £20 whereas normaly its fine to get one on the train or from a guard.
IIRC the ‘ticket inspector’ can’t give a penalty fare, an ‘official fare collector’ can, but they have to wear high vis jackets and show you ID then provide an invoice for you.
The stupid thing is, Reading has ticket barriers, so short of jumping the barriers right infront of BTP you can’t dodge the fare, it’s purely a money making excercise.