I have a London Transport 'Oyster' card for when I'm in London, very handy (Tube & Buses) and the inspectors can verify whether you've 'tapped' on or not from their hand held terminals, showing your current journey details in actual time. There's been a series on TV about Fare Dodging in London recently, which I thought demonstrated how effective and efficient this system was.
Much better than the horror of queuing for horrible ticket machines with lines of impatient pressurised people (time hungry, I think its called in Millenialspeak). You just 'tap on and off' at the screen terminals located by the entry and exit barriers, or access points on buses. You can pre-set your credit limit in advance (like a prepaid PAYG phone). Any unspent sums are just left pending. I wish they'd been around in my time of consistent 'Tube' journeys 50 years ago, or even twenty when they started shutting down or limiting 'ticket offices' everywhere.