Well, you probably saw this already, Railroad Bridge of
Recycled Plastic Installed at Fort Eustis, Virginia.
On May 10, Axion International Holdings, Inc. announced the completion of the first of two railroad bridges constructed from recycled plastic at Fort Eustis, Virginia. The bridges, which were designed by Parsons Brinckerhoff, employ Axion's patented Recycled Structural Composite ("RSC"), a thermoplastic solution consisting of nearly 100% recycled post-consumer and industrial plastic
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The new short-span bridges extend approximately 40 and 80 feet, respectively. Both bridges are designed to deliver a high-load rating of 130 tons (i.e. 260,000 pounds).
I found another site which stated the bridges were installed in 2010, so that matches with your rail speeder outing experience.
One more site:
A white paper describing in some detail the bridges (as well as other bridges) and construction methods, as well as the characteristics of the plastic used and so. Only one image of the railroad bridge, however.