After a bit of research, I came across a wikipedia page that explains what I’ve been calling “The Rust Box on Stilts” really is called & the purpose it served:
The 69th Street Transfer Bridge, part of the West Side Line of the New York Central Railroad, was a dock for car floats which allowed the transfer of railroad cars from the rail line to car floats which crossed the Hudson River to the Weehawken Yards in New Jersey. Its innovative linkspan design kept the boxcars from falling into the river while being loaded.
After it fell into disuse, it was in danger of being torn down and removed, but around the year 2000, during renovations of Riverside Park, following the example of Gantry Plaza State Park, it became a prominent feature of the park. It was listed on the National Register of Historic Places in 2003.
Taken @ Riverside Park South in NYC, using the native iPhone 4S camera. Edited in Dynamic Light & Camera+.