What is the main reason the pony express was a quick way to deliver mail?

Although it wasn't nearly as quick as trains when they were invented ,the pony express took a shorter time to mail letters  than ships,which would often take many months,but anyways the main reason it was fast was because at a certain point the first mailman could stop to take a rest while another would step in and go further and this would go on,and since horses have been known to be fast it was rather easier to travel.

Explanation:

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