Coins, cash, and disability-accessible faregates solve most dimensions of this problem -- portability, scalability, no vendor lock, security, privacy, and equal

Coins, cash, and disability-accessible faregates solve most dimensions of this problem -- portability, scalability, no vendor lock, security, privacy, and equal access -- but even that isn't universally deployed. If you want to support distance-based, or otherwise complex fare structures, have both entrance and exit gates demand a fixed payment, and give a one-time-use token, ticket, or rebate code that identifies the origin, which, when inserted at the exit, reduces the price if it's below the default amount. Non-payment at exit gates is a similar issue to being caught with fare evasion by roving inspectors, and has similar societal answers (or lack thereof). If you want to reduce entrance friction, you can additionally deploy faster payment methods for those who opt in, or reconsider the funding model of transit entirely. Roads are a similar public good, yet rarely are 'usage fees' deployed for roads; instead, the funding model of roads is complex, but fees designed to somewhat correlate with road usage, like fuel taxes, are deployed widely for this purpose. Both transit and roads are extensively subsidized from general government revenues as well. The effect is that roads are generally used without individual purchasing decisions needing to be expended for every trip, and together with cars' intrinsic point-to-point routing, it vastly reduces friction compared to transit. Rethinking the funding model of transit, in places where the road network is under strain and transit is a viable contender, would be worthwhile.