Columbia subject headings:
Harvard subject headings:
Historic sites--United States
Pony Express National Historic Trail.
Pony express--History.
Pony express--United States--History.
Postal service--United States--History
Trails--United States--History.
Princeton subject headings:
Historic sites--United States.
Pony Express National Historic Trail.
Pony express--United States--History.
Postal service--United States--History.
Trails--United States--History.