Public transit isn't sustainable either; it's about public good paid for by tax dollars. It usually costs far more to run buses in suburban areas than they bring in revenue.
IIRC, there were Amtrak routes in the Northeast that became very popular, and the gov't was bleeding money increasing the amount of service because every ride was subsidized by something like $10-$20/ride.
That seems rather unlikely. Generally, the reason public transport loses money is because it doesn't have enough riders, since the cost is relatively independent of the number of people that use it up to some point. You don't tend to get this kind of success problem in public transport, unlike with (say) VC-subsidised services like Uber.
IIRC, there were Amtrak routes in the Northeast that became very popular, and the gov't was bleeding money increasing the amount of service because every ride was subsidized by something like $10-$20/ride.