Which detail in the firsthand account best the readers picture how the pony express looked and sounded to the author

He came at a rattling pace.

Explanation:

In this excerpt, the author describes how the Pony Express looked and sounded to the author. We learn that the Pony Express was eagerly awaited by the crowd, who cheered as the express approached. We also learn that the band played with an inspiring air. However, the most important detail that describes the Pony Express is the fact that it came at a rattling pace.

thats exactly the right answer