I think some investigation and possibly review of this bridge's history is in order. I looked up old CB&Q system maps, and they do not show the line from Oskaloosa crossing at this point, but crossing at Tracy where the M&STL bridge was. I believe this bridge was a Rock Island bridge from the beginning. If you look at the 1930s aerial imagery, the line doesn't even go to Oskaloosa directly, but branches off the Rock Island's KD line northeast of town.
The CB&Q must've used the railroad bridge at Tracy, as the line joined the current day BNSF line to Des Moines here, and then went right into Knoxville. Some more research and review must be done on this bridge, too. I doubt the M&STL built the first Des Moines River bridge at Tracy. They probably simply shared trackage rights over the bridge with the CB&Q. The fact that the 2nd bridge at that location was built from pieces of a CB&Q bridge from Plattsmouth further hints to a direct connection with that railroad.
The fact that the given history of the CB&Q bridge doesn't jive with the bridge at Harvey further backs me up. Are they really referring to the first bridge at Tracy?
In addition to that, the date of 1878 is almost certainly incorrect. While it was not unusual for rail bridges to have been constructed of steel at this time, the portal style was commonly used by American Bridge Company and Lassig Bridge & Iron Works after 1885. It might not be impossible that it was built 1878, as all sources say it was, but I don't think that is the build date of the current bridge.
Very interesting, because this ties into some research I just did. The Rock Island built the second rail line that reached Knoxville in 1876, just one year after the CB&Q. I always wondered where the Rock Island bridge was. They must have decided it was better to tie into the CB&Q at some point and use their bridge, rather than building another one. The Rock Island might have retained use of the bridge longer than the Burlington did; they abandoned service in 1938, which is the exact year the bridge ceased being used for rail traffic.
The aerial imagery says this bridge was CRI&P back in the 30s.