Defecto “free” ridership has really hurt the local public transport system in the SF Bay Area.
Quite a lot of people refuse to take public transit due to risks of being harassed or witnessing open drug use. Easy to dismiss that as a guy personally, but I definitely think that creates an environment that’s hostile to many people who need to use or would otherwise be using public transit.
On the other hand, Margerite bus at Stanford has been free for decades, and it’s never been a significant issue there.
So it’s very difficult to generalize across different systems that have their own unique issues.