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.

(Pub. L. 91–375, Aug. 12, 1970, 84 Stat. 766.)