Today, the United Postal Service delivers mail by truck and by plane but it was not always delivered this way. In the past, mail was carried by stagecoaches and steamships. It was also carried on horses. For example, the Pony Express carried mail across the country to the gold fields of Sacramento, California in the 1860's. Around this same time, trains started to carry mail, especially with the completion of the Transcontinental Railroad.
Name a reason the Pony Express delivered mail in the 1860's.