39 U.S.C. § 5003 — Establishment of post roads

the waters of the United States, during the time the mail is carried thereon;

railroads or parts of railroads and air routes in operation;

canals, during the time the mail is carried thereon;

public roads, highways, and toll roads during the time the mail is carried thereon; and

letter-carrier routes established for the collection and delivery of mail.