Once upon a time, Austin, TX, made its Capitol Metro busses free. Ridership went way up, although there were a couple of complaints. The first was that really sketchy people starting riding the busses a lot[1]---possibly because they were heated/air conditioned---and the second was from UT Austin students, who were still paying for bus service as part of their student fees.
I never heard why they stopped, though.
[1] At one point, I was waiting at a bus stop and a man who was either on serious drugs or schizophrenic came up and started talking to me. When the bus arrived, he followed me on but didn't sit next to me while I started reading a book. A few minutes later, he started behaving even more oddly and pulled out a crack pipe. Sheriff's deputies eventually removed him because he was getting threatening.
I never heard why they stopped, though.
[1] At one point, I was waiting at a bus stop and a man who was either on serious drugs or schizophrenic came up and started talking to me. When the bus arrived, he followed me on but didn't sit next to me while I started reading a book. A few minutes later, he started behaving even more oddly and pulled out a crack pipe. Sheriff's deputies eventually removed him because he was getting threatening.