Mail by Bus

Highway Post Office service was initiated in response to declining railroad traffic in the early 1940s. As the American population grew, use of the highway system grew as well, meaning fewer passengers traveled by rail. This, in turn, meant fewer trains available to carry mail in several parts of the country. The service grew very slowly at first, not really taking-off until the 1950s, when these brightly-colored buses became common sights on American highways.
Read more at:
npm.si.edu/exhibits/current/mo...