Between 1905 and 1912, Henry Flagler extended his Florida East Coast (FEC) Railway to Key West through the massive Overseas Railroad project, which required 23 bridges using innovative technology. The 6.7-mile stretch between Knights Key and Little Duck Key was the longest over-water expanse and used steel girder deck spans, 9,000 feet of concrete arch viaduct, and a metal truss swing span. The bridge crossed over Pigeon Key, which served first as an FEC staging area during construction and later as a permanent FEC camp. Historically known as the Flagler Viaduct, it was also called the Knights Key Bridge, the Seven Mile Bridge, and later the Old Seven Mile Bridge. After the Overseas Railroad was severely damaged by the deadly 1935 Labor Day Hurricane and abandoned, the State of Florida bought the Florida East Coast Railway right-of-way for $640,000 in 1936 and converted the rail bridges for vehicles between 1936 and 1938. Because the bridge had narrow foundations, engineers installed new metal I-beams to support the roadway, and the FEC rails were reused as guardrails that remain visible. Completion of the Overseas Highway on March 29, 1938, made it possible to drive from Miami to Key West without taking a ferry. The bridge was listed on the National Register of Historic Places in 1979 for its engineering and transportation significance in Florida, was replaced by a new bridge to the south in 1982, and in 2021 about 2 miles of it to Pigeon Key were rehabilitated.