Once, these two, mail and rail, were an item. In many ways, they were inseparable, at least if any significant distance was involved. Mail needed rail both to get where it was going, at least beyond local office limits, and to arrive at the destination in a timely manner, i.e. faster than the Pony Express.
Rail, passenger version, depended on the mail to provide an economic underpinning for many services. When the death throes of the RPO (Railway Post Office) occurred in 1...