The Pony Express did its thing, delivering mail from California to Missouri (and vice versa) from April, 1860 to October 1861, a total of 18 months. For its day, this was snappy service; mail made the trip in 10 days or under. Whooosh!
What short-circuited the whole business was the Transcontinental Telegraph, which worked even faster. Technology, technology.
Still, there is a great romance to Pony Express riders. Please recall: few Rhinos get mail of any kind whatever.