Well it was more than that. Air travel became more common and less expensive. The federal highway system was built. The rail infrastructure was old and needed repair or replacement. The railroads vastly over-expanded and could not afford to maintain lightly used tracks. The railroads also had purchased many new passenger cars and recently replaced all of their steam engines with diesel electrics. With the increase in rail traffic during the war the railroads never forecast that they would have competition between cars and airplanes. It was kind of a perfect economic storm. These were all companies that had been in business for 150 years surviving all sorts of ups and downs. These railroads all merged dropped passenger rail and still went bankrupt. U.S. mail was a hit and airplane travel didn't help either. But the federal highway bill was the death knell. Another government boondoggle.
By the way long haul trucks do the most damage to the highways. If you eliminate the trucks from the highway system then you will extend the life of the highway. But you also lose the tax revenue.