Is there a plaque on the bridge? That might provide a clue if it did. Here is a turntable bridge had a unique plaque that listed patents and other info that would not be found on a deck plate girder bridge: http://www.historicbridges.org/michigan/turntable/
Actually, looking at the shape of the ends of the bridge and the overall size in Photo #1, this looks more like a salvaged roundhouse turntable! That would be really cool to see, but some verification of this would be required. If anyone is able to find out if this bridge is either of these, please post! :>)
I read recently where someone claimed this bridge was actually an old hopper car converted for bridge use. I haven't had the time to get back there to investigate now that the trail is finished. Anyone else gets the chance, check it out and let me know!