This bridge was part of the Rock Island's former KD line from Keokuk and Des Moines. This was originally built by the Keokuk & Des Moines Railroad before it was taken over by the Rock Island. The line south of Pella was abandoned and torn out in the early 1980s following the Rock Island's shutdown.
I find it very interesting that this bridge was left in place and abandoned without been demolished. There are also a lot of wooden bridges left in place on the southern half of this line.