39 U.S.C. § 5003: Establishment of post roads

The following are post roads:(1) the waters of the United States, during the time the mail is carried thereon; (2) railroads or parts of railroads and air routes in operation; (3) canals, during the time the mail is carried thereon; (4) public roads, highways, and toll roads during the time the mail is carried thereon; and (5) letter-carrier routes established for the collection and delivery of mail.