This is the Rock Island's Choctaw Route, abandoned in 1981.
I watched Rock Island trains here in the 60's and 70's. What kind of FOOLS abandon a railroad that parallels I-40 for a thousand miles? How many trailers could be moved faster, cheaper, and safer?
Looks like a thru plate girder that someone's cut out the panels. Seems like there's another bridge or two on this site that's been partly salvaged the same way. What's left would not hold nearly the weight that the floor structure could.
This is a bizarre bridge! It may not be a true example, but it reminds me of a Vierendeel truss.