§ 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.