There are transit systems with >100% farebox recovery (i.e. the fares they collect cover the entire operating cost of the transit system), but this usually requires very high ridership. The New York subway’s farebox recovery rate is around 25%.
Usually systems like this do in fact have fare enforcement - but instead of using expensive fare gates at every station, they rely on roaming fare inspectors, who will walk through trains on a random/unpredictable schedule, checking each passenger’s ticket and issuing citations to anyone who doesn’t have one. The fine is set sufficiently high to make up for the low probability of encountering a fare inspector on any given trip: if you take enough free rides, you’ll eventually get hit with the fine and end up paying more than if you’d just paid the fare. This approach is called “proof of payment”.
This can save a lot of money by replacing expensive faregates (with many moving parts, motors, and sensors) with a simple tap-on-tap-off terminal, and is also great on bus systems where it can enable all-doors boarding, speeding up high-traffic bus routes significantly.
