Urban mobility is just one tier in the structure of o mobility restriction. As the mixing of the classes is restricted, so is the inter-city flow. However fare economics are quite different. Train travel becomes less and less attractive relative to air fare. Given the high economic, social and environmental costs of air travel relative to train travel this hardly can be a factor of funding cost. A few differences come to mind that relate to control. Train stations are in urban centers. Airports are in remove locations. Trains tickets often have variable travel times, as well as well as optional routes and transfers. Airline tickets, at least the cheap ones, are for one specific flight with fixed connections. Train stations have casual security and require no ID. Airports have strict security and require official ID. Once again fare prices make are better understood from the perspective of control than provisioning cost. Authorities have a lot more information on when and where the air traveler is going than the train traveler, and a lot more control.