A public transport bus usually has several entrances, you can have a machine inside the bus at each entrance. Not everyone will play nice and pay if the bus dri

A public transport bus usually has several entrances, you can have a machine inside the bus at each entrance. Not everyone will play nice and pay if the bus driver doesn't check the ticket, but you could have random controls on the bus from time to time. In practice you of course have to weigh the costs of these different approaches.