If GTR, like FCC before it, ever intended to go to court and prove the ticket was tampered with then I believe it would require a proper forensic investigation, including the ticket being taken and handled with extreme care. Bagged up and sealed in the first place.
Once an RPI takes the ticket and tries to demonstrate how the ticket was tampered with, perhaps by rubbing his/her finger over it, or picking out where the paper has been damaged, all evidence is probably destroyed in terms of what would be required in court.
Therefore I'd say it would be hard to bring something to court, or get a successful prosecution (assuming someone didn't just plead guilty) and it will rely solely on threats of prosecution to secure a nice out of court settlement, which seems favourable anyway.
I do think the carnet tickets should be axed, or replaced with the scratch cards that used to be given out for promotions/compensation/season ticket holders. Once marked, you seal them with the plastic cover and that's it.