First, letters were generally much smaller, and there were very few periodicals, advertising circulars, catalogs, or parcels. That meant a single rider on horseback could carry the mail of a small city by himself, and post roads were among the first cross-country land routes established in the US. They’re specifically mentioned in the Constitution.

Later, packet boats and horse-drawn coaches were used to carry mail to the expanding frontier of the Old Northwest. Stagecoach lines had mail contracts that covered most of their expenses; passengers who also took the lines provided extra profit.

As railroads expanded across the nation, they began to carry nearly all intercity mail. The famous Pony Express only lasted 19 months until the transcontinental telegraph line was complete and it was soon supplanted by the transcontinental railroad. Again, for decades, many passenger trains were largely supported by—and run at the times they were to suit—contracts to carry the mail. The Post Office’s decision in 1967 to shift nearly all first-class mail to air and trucking was a big reason for the downward spiral of passenger trains in the US.