I just completed a Model Structures Company 80 Foot Straining Beam Truss Bridge kit. These kits were from the 1940s, but the craftsmanship is equal to any contemporary kit. The kit consists of pre-cut, machined, and drilled pine pieces, metal castings, threaded brass rods, and hardware. The template is a blueprint taken from the Southern Pacific prototype blueprints. The kit was a delight to build due to the pre-cut machined wood. I bought the kit used, with one truss partially constructed and broken, so I needed to fabricate some missing wood braces and threaded rods, and buy some 0-80 nuts.
The rail is Atlas code 148. I'll be spiking down an N scale rail for the center rail (thanks Ron H and Marker).
Here is a 1941 news article about Model Structures Company.
Another forum member has one of these kits available. Search for Model Structures Company.
Bob