It was purchased by the state of Florida and converted for highway use in 1938. Rather than rebuilding the bridge, the top of the structure was redecked for use as the Overseas Highway (the existing deck inside the truss was too narrow for vehicular traffic).....
Before being reused as a highway bridge, the Bahia Honda Rail Bridge carried a single track of FEC across Big Spanish Channel from Bahia Honda Key to Spanish Harbor Key. Unlike other bridges on the Overseas Railway, the Bahia Honda has a steel truss construction]This was a necessary difference from the predominate concrete arch form of the other bridges of the Overseas Railroad, as the channel is the deepest of those spanned, at 24 feet.
See the Wikipedia article for more information. The picture and video below make it easy to see where the original rails were and the highway deck.