(in the US) a system of mail delivery operating from 1860–1 between St Joseph in Missouri and Sacramento in California, using continuous relays of horse riders.
post, letters, packages, parcels, correspondence, communications, airmail
Definition of Pony Express in US English:
Pony Express
proper noun
A system of mail delivery operating from 1860 to 1861 over a distance of 1,800 miles (2,900 km) between St. Joseph, Missouri, and Sacramento, California, using continuous relays of horse riders.