Why is she in Panama again?
Months before her wedding she is in Panama?
Was she a spy or was she doing her semester abroad?
She was en route between a point of origin that I don't recall and England. Even though there was not yet a canal, the quickest way to cross from the Pacific Ocean to the Atlantic in sailing days was to debark a ship on one coast and go overland to the opposite coast. They eventually built a railroad. I don't know if it went between Panama City and Colon or two other points.
The original book goes a little deeper into explanation, but not much.