Horses are extremely fast--but they can't run at top speed for very long or they overheat. The only way to ride at a full gallop over long distances is to regularly switch to a fresh horse. For most of human history, this was the fastest way to transport people and parcels over land.

The fabled Pony Express of the American West is the most famous horse-based relay system, but it was not the first, the largest, or the most successful. Such relays date back nearly 4,000 years and were used widely in ancient Babylonia, Persia, China, Mongolia, Egypt, Italy, and France.

Pony Express letter carrier, United States, Modern reproduction.

Denis Finnin/AMNH

Pony Express

In 1860, there was no coast-to-coast railroad across America. The fastest way to get a letter across the country was still by horse-drawn stagecoach. This took 25 days or more--faster than sailing around South America, which took at least 45 days, but still pretty slow. So in April 1860, a new postal service called the Pony Express was born. With railroads handling the eastern leg and horsemen racing day and night from Missouri to California, the Pony Express could get a letter from coast to coast in just ten days for five dollars, later reduced to one dollar.

Relay Race

Of course, no single horse could carry mail day and night without rest. So a series of relay stations was spaced 16 kilometers (10 miles) apart, and riders passed a small bag of mail from horse to horse, like sprinters passing a baton. Some riders supposedly blew a horn before reaching each station, so a fresh horse would be waiting for them to jump onto the moment they arrived.

The Legend Grows

The Pony Express opened in April 1860--and closed in October 1861, a few days after cross-country telegraph became available. Though the Pony Express went out of business just 18 months after it began, its fame only continued to grow. In the late 1800s, traveling shows such as "Buffalo Bill's Wild West" included demonstrations of Pony Express riders leaping from one horse to another. The image of brave riders racing across the open range became a symbol of the American West. Today the U.S. Postal Service uses the Pony Express as its symbol.

Blazing Saddles

Within two minutes of arriving at a station, a Pony Express rider left on a fresh horse. To speed up the transfer, mail was carried on top of the saddle in a removable leather overlay, or mochila. At each station, the mochila was slapped onto the saddle of a waiting horse. Like many ranching terms, including lariat, lasso, chaps, and ranch, the term mochila comes from the Spanish language.