You may be right there. I've noticed in the short time I've looked at them that Rock Island track charts cannot be fully trusted on their information. Perhaps the 1960 flood damaged both ends of the bridge, necissating the DPG span addition on one end and causing the loss of a TPG span on the other.
One thing I can tell you sure; there has definitely been a bad derailment in the area at some time in the past. There is at least 3-4 railcars worth of trucks laying around in the swamp, and knows what else is buried in the area or was taken out in the original clean up operation.
I think the unused stone pier under the approach is from the flood of 1960 (when the girder was built), as that flood created problems in southern Minnesota and northern Iowa. Just a guess.