The cost of using any transport conduit is principally the journey time, whose value usually far exceeds the ticket price.
Time, after all is money.
The marginal cost of using a given conduit refers to the cost for each additional traveller.
Rail arose first, and exhibits falling marginal cost with flow. Mechanized road transport arrived later, with a rising cost curve. While some migrated from rail to road, the two might have remained in stable competition.
Then the State invested heavily in new roads, while burdening the railway with heavy regulation (on both sides of the Atlantic).
The effect was modal inversion, which left both modes considerably more expensive to use.
(Click here for an animation, and keep clicking.)
To reverse modal inversion will be very expensive indeed.