If as you say the bridge was sitting quite a bit higher than the adjacent ground, it may indicate that the now-gone approaches were a berm -- which would further indicate it was a railroad bridge on a now-abandoned ROW. Wagoner was a transfer point from the old Missouri Pacific main line extending northwest out of Van Buren, AR via Wagoner and Claremore; that town was where it interchanged with the "Muskogee Lines", which were three interconnected regional rail lines
(Kansas, Oklahoma, and Gulf; Midland Valley, and
Oklahoma City-Ada-Atoka). KO&G was the main "trunk" of that group, extending south from Kansas all the way to Texas; a map can be found at:
www.abandonedrails.com/Kansas_Oklahoma_and_Gulf_RailroadThe map indicates that KO&G had at one point multiple lines in and around Wagoner; this bridge could well be part of one of those alignments. Parker trusses were common on the Muskogee Lines, as they were less expensive to construct compared with most box truss designs (and, if anything, the management of those lines were said to be "penny-pinchers"). And seeing as how Wagoner is in an area where flooding is certainly not unknown, there's a possibility that the berm was washed away at some point, with the line being subsequently relocated to a less flood-prone location -- but the bridge remained until, likely, at some point someone "salvaged" the steel from the deck. Of course, all this is speculation -- but when Wagoner was mentioned, it rang a bit of a bell with me. Do feel free to continue to check out all archival info -- but be sure to include the annals of the Muskogee Lines in the process; they might well be relevant to the search for bridge info.