San Antonio-San Diego Mail Line

The San Antonio-San Diego Mail Line, also known as the Jackass Mail, was the earliest overland stagecoach and mail operation from the Eastern United States to California in operation between 1857 and 1861. It was created, organized and financed by James E. Birch the head of the California Stage Company. Birch was awarded the first contract for over land service on the "Southern Route", designated Route 8076. This contract required a semi-monthly service in four-horse coaches, scheduled to leave San Antonio and San Diego on the ninth and the 24th of each month, with 30 days allowed for each trip.

Birch envisioned that at New Orleans, one could take a five-times-a-week mail steamer to 540 miles to Indianola, Texas. There one transferred to a daily line of four-horse mail coaches traveling 140 miles to San Antonio, Texas. Then one would take the San Antonio and San Diego Line 1,476 miles from San Antonio via the San Antonio-El Paso Road and then continue north to Mesilla and take the Southern Emigrant Trail from there to San Diego. Once on the Pacific Coast the passenger could board a California Steam Navigation Company vessel to San Francisco.

To accomplish this Birch entered a partnership with George H. Giddings, of the San Antonio-El Paso Mail that already ran over half of the route to La Mesilla. 87 watering places and stage stations were organized by Superintendent Isaiah C. Woods, formerly of Adams & Company of California in San Francisco. On the first mail run, they were setting up the line as the mule trains and coaches journeyed west from San Antonio. Superintendent Woods prepared a self-contained outfit for this journey across the unsettled country of Texas, New Mexico Territory and Southern California with almost no existing infrastructure. The vehicles used were celerity wagons or mud wagons, also called ambulances, (which was the military use for the same type of vehicle at that time), rather than the better known Concord stagecoach.

...
Wikipedia