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.