Perhaps the most famous (or infamous) abandoned railroad in the US, the Key West Extension of the Florida East Coast Railroad was completed by 1912 and was considered a miracle of modern engineering. The "railroad that went to sea" traveled over a large number of bridges in the 127 miles from Florida City to Key West, the longest one spanning 7 miles. The Key West Extension was the dream and brainchild of Henry Flagler. He lived to see the first train arrive in Key West, then died soon after.
In its heyday, the extension saw numerous trains of passengers bound for Havana, Cuba via boat from Key West. However, the line was plagued with hurricanes and slow speed limits over the bridges. Then, in 1935, the great Labor Day Hurricane tore through the middle section of the line and completely destroyed it for miles. Hundreds of lives were lost, many of them on a train full of World War I veterans who were being evacuated from a work camp. The cost to rebuild the line was too great, and Florida East Coast sold it to the government. An engine and consist stuck in Key West had to be brought back to the mainland by boat. The government then converted the abandoned ROW and bridges into US 1, extending this highway all the way to Key West. The train bridges had to be modified to accomodate two lanes of traffic, and the roadway actually traveled along the top of the truss bridge at Bahia Honda (see picture below). The highway bridges were used until 1982, when new modern bridges were opened along the entire length of the Keys. However, many of the bridges still remain in place, and a portion of the 7-Mile Bridge is still used to provide access to Pigeon Key (see picture below). This certainly has to rank towards the top of a list of most interesting abandoned railroads in the US.
This Railroad bridge is very visible on google earth and has a street view looking at it from the new u.s route 1.
These bridges are so interesting. In 2009, I took a walk on the section of the old seven-mile bridge that connects to Pigeon Key. I thought it was neat how the old rails from the railroad tracks were turned in to guardrails on the bridge.