Many cities in North America do. The key is the fine for not having a ticket is high enough that you are on average much better off having a ticket. Generally this works out to enough random inspections that the average person is checked once a month, and the fine for not having a ticket works out to the cost of a 3 month pass. The exact numbers are of course subject to debate, but the above should give any city a good starting place they can play with.
IMHO, if you have fare gates they need to be tied into a parent control system so that parents to limit where their kids are allowed to go alone. I've never seen the implemented and the details are important to get right.
Where I live, I'd be far better off not buying the tickets. The fine is less than 2 months with a pass and I'm checked 2-3 times a year. Yet most of these checks don't find anyone without a ticket. Monthly ticket costs about 1.5% of average monthly income for that city, less than 4% of minimal wage. I'm quite convinced that reasonable pricing is the key.
That's probably a significant part of it. Also accessibility of the monthly passes. I used to live near a rail stop in Tempe/Phx area, and would use it when I had to go to the airport or to Downtown Phx as it was easier than dealing with parking. The ticket kiosks were a bit of a pain, but easy enough, widely available and not overly expensive.
I didn't use it that much, but did see ticket checks on one of the trips, nobody was without one.
IMHO, if you have fare gates they need to be tied into a parent control system so that parents to limit where their kids are allowed to go alone. I've never seen the implemented and the details are important to get right.