39 U.S.C. § 5003

(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.