Instead of selling tickets and passes directly, sell bonds that pay interest in tickets / passes: you pay $1000, you get ten rides per month. These bonds would be transferable. That way, it's restricted to people who can put together, say, $1000 in cash. It wouldn't be hard to borrow that much, if you didn't have it on hand, unless you were a total failure.<p>Tourists would just put up the money as a deposit, and get it back at the end of their trip.