This UP line through South St. Louis originates from the BNSF railroad west of Vandeventer/Tower Grove, travels west of Tower Grove Park, and continues through to meet up with the UP main line that parallels the Mississippi River at a point just north of the River Des Peres. Amtrak also uses this line, UP freight trains use it about five times daily.
Is this a main line?
Google books has a couple of listings for the UBC around this time at 1 Broadway and 100 Broadway. Looking at the picture of the plaque it might read "No.1 Broadway".
I think that it was bulit in 1921 and the first bridge was built in 1856
It is a safe bet to say this bridge was constructed by the Union Bridge Co. of New York in 1894.
I made a personal visit to this bridge on April 11th. I was looking at the builders' plaque with binoculars and was able to discover that the bridge was built by the Union Bridge Co. of New York. It looks like it was built in either 1894 or 1934, but the year is partially obscured by so many varying shades of color and flaking paint due to the railroad logos painted on the bridge.