I’d often noticed this strange structure while walking around at the site of the former Rockwood Hall. For some reason I seem to have spotted it in Summer when it is largely obscured by leaves so I couldn’t find a good place to take a picture. This time the leaves were off the trees and I noticed a path that allowed me to scramble down closer to the railway tracks. From there I could get a better view even if it did mean that I slipped a couple of times going down the rather steep slope.

As you can see it spans the railway tracks. Less clear from this angle is that the platform on top is covered in wood planks. It’s not connected to anything on either side, but I suppose it must have been at one time.

Update: I’ve just discovered from The History of Rockwood Hall that this used to connect to the boathouse on the other side of the metro north tracks. Slide 12 provides a drawing and this description:

The estate had a sumptuous boat house and substantial dock. In the early days, the estate had its own electric lighting plant. It also had a spring which provided about 75 gallons of drinking water per minute. A siding was added to the New york Central Trcks along the Hudson River, where Rockefeller kept his private railroad car. Near the ansion was a stone ice house with a capacity of 400 tons.
…
The boatshouse was reached by a 150-foot steel bridge of the New York Central Railroad. About 50 feet upstream was a dock which could accomodate a 700-ton coal barge.