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.
Of course all those factors contributed to the decline of passenger service. That process began long before freeways were built. The switch to diesels began three decades years prior to the creation of Amtrak, and they were cheaper and more efficient to run than steam locomotives. The Feds would not allow some roads to drop unprofitable routes, and the increasing danger of a catastrophic lawsuit made the passenger market more and more risky. But the death knell was not the highway system, but the end of revenue from mail service, which took place in 1964. Railroads like Santa Fe wanted to keep going with profitable routes like the San Diegan and the Super Chief, but without those revenues even they were money losers. Even at that, six railroads refused to join Amtrak, but eventually the Feds brought enough pressure to bear and Amtrak was born.
What is so bad about the Highway system?? It allowed Americans to move more freely than before..