Mail service running the 1,840 miles from St Joseph, Missouri, across the Great Plains, and over the Rocky Mountains and the Sierra Nevada to Sacramento, California, by horseback, using relay stations. At Sacramento, messages were placed on a steamer and sent down the Sacramento River to San Francisco.
Missouri, Kansas, Nebraska, Colorado, Wyoming, Utah, Nevada, California. Lake Tahoe is shared between the last two.
During its eighteen months of operation, April 3 1860 to October 24 1861, it reduced the time taken for messages to travel between the Atlantic and Pacific coasts to about ten days. It was a precursor to the First Transcontinental Railroad, constructed 1863-69, and ceased to operate when the Civil War broke out.
Map by William Henry Jackson, reproduction issued by the Union Pacific Railroad Company in 1960 to commemorate the 100th anniversary; Denver is 200 miles south of Fort Laramie, Wyoming, and not on the route; Library of Congress and Wikipedia