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.