If not subways, then proof of payment systems are definitely the norm for light rail and busses around Europe. Though whether these systems are based on trust

If not subways, then proof of payment systems are definitely the norm for light rail and busses around Europe. Though whether these systems are based on trust depends on the ticket price and the nature of the checks. My DB train tickets were checked on about 50% of the trips I took in Germany, meaning it was in my economic self-interest to pay the expensive fare every time.