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.