Why and how did people quit using them? Suppose you're in a world where the interstate highway system hasn't been built and subway trains are the dominant mode of transportation, and you would like to stop taking trains to get to places, what do you do? Walk? Bike? Ride a horse? Drive offroad in a car with a manual transmission and no seatbelts or airbags, with no gas stations in between?

Seems like you'd be S.O.L. unless the government kindly free-markets a continent-wide network of paved roads, with conveniently placed roadside stops along the way where they sell gasoline. Can't imagine why they would undertake such an exorbitant public works project when the transcontinental railroad already exists; it's not like the government is disproportionately influenced by any industries that would very obviously materially benefit from such a thing.