There is an interesting, very American, solution, which is ubiquitous, active surveillance on every form of public transport. Not just for violent threats, but for enforcing (current) vandalism, littering, and even assault laws cheaply and at scale. For the low low cost of a dystopian panopticon you could have, theoretically, a cheaply maintained public transportation system. (The other option, which many other countries employ, is a rigorous culture of respect for public places and public facilities -- for example Japan and Northern Europe.)