39 U.S.C. § 5003

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