If this was really a railroad bridge I think it would be hard to even consider the Pratt a 1909 structure, let alone the Whipple. By 1909, railroad bridges were far more massive than this. Could this be a bridge relocated and reused in this place in 1909?
Either way, an excellent find, I was unaware of this rare bridge in DC.
This bridge looks to be much older than dated.
The Pratt could be 1909 but the Whipple would be pre-1900.
Bridge has a curve built it. Not sure of the builder though. Here is a image from the deck where you can see the curve. http://www.flickr.com/photos/mvjantzen/248793397/