As I said in another comment, I've used US systems where you board the vehicle (bus/train) before paying, and bus drivers wave you past if you can't pay. On the train, you get a free ride to the next stop.
On the Phoenix area light rail the only way they (used to, at least) tell if you have a valid ticket is random security patrols checking everyone on the train. No gates, no nothing.
During covid they even stopped checking the validity of the tickets and all you needed was to be in possession of 'a ticket' -- I used the same one for a couple years and still have the thing in my wallet in case I ever go back there again.
Couldn't even begin to count the number of times I saw people get off the train as soon as they saw security get on and just wait for the next train.
They went to a stored value card in the last year or so. It's the ugliest one I've seen (big ugly UPC code on the front, I guess maybe for convenience to sell them at retailers outside the transit stations)
During covid they even stopped checking the validity of the tickets and all you needed was to be in possession of 'a ticket' -- I used the same one for a couple years and still have the thing in my wallet in case I ever go back there again.
Couldn't even begin to count the number of times I saw people get off the train as soon as they saw security get on and just wait for the next train.