Mark,
The bottom of the river bed is around 46 or so inches. This is so I can duck under fairly easily. The N&W tracks will be three or four inches higher. The top of the bridge is about seventy five inches. This is so you look up at the bridge as you would have when it was standing. As I said Google Glen Lyn VA and look at the images. It is a truly impressive bridge. It also had catenary on it.
George