Mr. Flagler was involved in the founding of Standard Oil in a partnership with John D. Rockefeller. This, in the decades before gasoline was even a thing, meant that they eventually created a monopoly in oil and kerosene manufacturing and sales in the United Stated. At one point, they were refining over 80% of the entire oil production in the world. As you can imagine, it was a not a always done in a friendly atmosphere, but they got extremely rich.
Sometime in the 1890s, Mr. Flagler became interested in development in Florida. Hotels specifically. Then a railroad to bring clients to those hotels. Then more hotels and further extensions of the railroads. He named Miami. By 1896, the line had reached Biscayne Bay. An incredible accomplishment for a visionary man by any measure.
His imagination was sparked again by the opening the Panama Canal and he decided to extend the line across 113 miles of open water and islands through the Florida Keys. The line was completed in 1912. It was called the Eighth Wonder of the World*.
The investment over a 30 year period was $50 million (in unadjusted dollars) in railroads, hotels, and infrastructure. It opened up Florida for tourism and agriculture. The extended line was never profitable, although it continued to operate until 1935.
In 1935, a category 5 hurricane hit Florida on Labor Day*. Many of the Florida Keys were completely overwashed by the storm surge. The loss of life is estimated at between 400 and 500, but that is an estimate. The railroad was damaged along most of the line, with 40 miles of tracks completely destroyed. The last time the train ever ran was the day the hurricane made landfall. Bankrupt and unable to rebuild, the remains of the line, including all the overwater bridges, was sold to the state of Florida.
The WPA got involved and rebuilding the line to Key West as an automobile highway was undertaken as a public works project. Completed in 1938, and upgraded several times over the decades, the highway still runs generally along the path of Flagler's Folly.*
But that any man could have the genius to see of what this wilderness of waterless sand and underbrush was capable and then have the nerve to build a railroad here, is more marvelous than similar development anywhere else in the world. --George W. Perkins, speaking of Henry Flagler after his death
*I highly recommend the links and then a Google search of your own. This was a very abbreviated post on the topic.