U.S. Hist. 1. a government mail service, started in 1848, for sending mail from the Mississippi to the Far West.
2. (caps.) a stagecoach line, established in 1858, linking Memphis, St. Louis, and San Francisco, which was then paid by the government to carry U.S. mail to the Far West. With various changes in ownership, name, and routes it continued until the completion of the transcontinental railroad in 1869.