The Main Street overpass at the south end of Ste. Genevieve consists of a pony plate girder bridge and a wood trestle. Taking measurements from Google Earth, I estimated the girder bridge span at about 75′ and the trestle at about 136′. More information and images can be found at the excellent Bridgehunter website.
The bridge was reduced to 50 scale feet and built with Evergreen Styrene parts. MicroMark resin decals were used to simulate rivets. The critical factor for me in reducing the scale was to retain the extremely low slung appearance of the original. If one looks closely at the Google Earth image, the clearance sign indicates only 9 feet from road to bridge!
Much of the bridge fabrication followed practices I found while perusing the 1906 edition of Details of Railroad Girder Bridges by Clifford Suttle. A PDF can be downloaded from the University of Illinois.