> Does a paper ticket do anything to prevent this?
Ticket inspectors used to mark the tickets so they'd know if someone was reusing a ticket. Originally it would be clipped, then date stamped, and towards the end on South West Trains it was a scribble of biro (or black marker if you were unlucky).
On the occasional trips I take I now use an e-ticket. I have never had the QR(ish) code on it scanned for validity.
London → Clapham Junction → many more stops → Hassocks → Brighton.
You could buy two paper tickets, London to Clapham Junction and Hassocks to Brighton. Use the first ticket to enter in London, the other to exit at Brighton. This only works if you're confident the ticket won't be checked on the train.
A safer option: buy an open return ticket London → Brighton. The London→Brighton bit is only valid that day, but the Brighton→London bit is valid for 30 days. Get through the barriers at Brighton with a Brighton→Hassocks ticket. Show the return ticket as required to the ticket inspector on the train, and use a Clapham Junction→London ticket to exit the station (the barrier swallows this used ticket).
The first of these is known as “doughnutting” and train companies are cracking down hard on it - there are countless reports of prosecutions on the busiest UK rail forum. They use a combination of ticket sales analysis (because most people do it with the same card and through the same retailer) and CCTV.
> The London→Brighton bit is only valid that day, but the Brighton→London bit is valid for 30 days. Get through the barriers at Brighton with a Brighton→Hassocks ticket.
Wait, if you already have a Brighton→London return ticket, why would you bother to buy Brighton→Hassocks and Clapham Junction→London tickets? Couldn't you just use the Brighton→London return ticket?
This way, you can re-use the return part of the ticket for multiple trips, either until it gets stamped or marked by the ticket inspector, or the 30 days are up.
