I agree; it appears to be a timber slab bridge. These timber slabs have been actually used up in Minnesota for new bridges with some regularity recently; the new CR668 bridge in St. Louis County, which replaced a Warren pony in 1988, utilizes this very same design. I'm not sure why they used this type, but they certainly look a lot nicer than a nasty UCEB.
This bridge isn't much to look at, but I think it might be a rare timber slab bridge. It does not seem to have any spacing between the stringers and it appears as though the members have been bolted together near the ends and rail points in order to act as one load carrying unit.