Transit cops boarded the train I was riding home after a morning errand and as one of them headed down the car to check fares she spottted and picked up a can from an empty seat as she passed, muttering about people leaving their trash ... then suddenly she suddenly had a huge grin on her face and "Cool! Free beer!" Yep, somebody had dropped an unopened can from pocket or bag. Her teammates were jealous ... guess tips are pretty rare in that job. ;p
The trains here are on a kind of honour system ... there's no gates or anybody checking tickets as you board. Instead there are random spot checks by the transit cops and you have to be able to show them proof that you've paid your fare (date/time-stamped ticket or daypass or a monthly pass) when they board. The fines for failing to be honourable are quite steep.