There's a plaque on the northern most approach span from American Bridge Co 1920. possibly just for that one deck plate span over Rockwell Rd.
This bridge hasn't been operated by the Illinois Central for many years. It was on the I.C.'s original charter line from Galena IL. to Cairo IL., nicknamed "Gruber line", that was abandoned from Freeport to Heyworth in 1986, the last I.C. train ran on this line on December 21 1985. The bridge, and a few miles of track south of the river are operated by Buzzi Unicem (formerly Lonestar Cement) and are the only section of the Gruber Line remaining. The north end terminates at the remains of I.C.'s LaSalle yard and connects with the CSX/Iowa Interstate (former Rock Island) tracks that pass underneath it. Quite a few lost and abandoned bridges can be found along the 100+ mile length of the line.
Here's a link with some pics of the bridge in the I.C. days: http://icrr.tripod.com/id3.html
I need to double check dates. The drawings I have were done by Illinois Central RR. They could have been done after construction. I also have some of the orignal Chicago Bridge & Iron "Bills of Wrought Iron/Steel" for components of the 1896 deck trusses. The word iron is crossed-out and replaced by the handwritten word "steel". The dates in these Bills of Wrought Iron/Steel should give an indication of date of construction.
I have the original drawings of this bridge (ink on linen). The deck truss section drawings are dated 1895 (built in 1896). The arch long span was built in 1932 to replace two 1896 sections, one of which was a draw or swing bridge. The piers were built in 1851 for a previous bridge and upgraded in 1896 and 1932.