A full airliner has reasonable fuel per passenger mile (better the farther the distance). Considering the pricing of bus, train and plane travel has a large fuel component, most airline routes are competitive with trains although not with buses like the China town buses driven by unqualified drivers.
Also realize this: If a person were to travel 2,500 miles across the nation by car, they would spend a whole lot in refilling the gas tank. Possibly more than a plane ticket.