There are two options, either make sure sure everyone who should pay is paying within an acceptable margin of shrinkage (i.e. theft), or cover the entire system publicly and make no one pay.
As for the 506, nearly every time I get on that car I find some insane guy muttering to himself who stinks of piss. I can't imagine the driver gets overly interested in who's paid, and instead just wants to get done with his shift without getting assaulted or having to deal with nutbars attacking those in the back.
My grandfather was a bus driver in London, UK. Their solution to ensure payment was to have an inspector/fare collector on the bus. No one got on without paying. Perhaps everyone paying their fare will cover the extra cost of the second staffer on the bus.
https://en.wikipedia.org/wiki/Manual_fare_collection