I've linked to a 1938 document that contains a bit more info about this bridge and a lot of other interesting information about the 1930's Express Highway through St. Louis and its construction.
Looking at the map with the text below and tracing the current Papin Ave, this bridge would have been right where the Kingshighway ramps merge with Highway 40 east of the interchange.
There is a picture of this bridge on page 21 of the PDF, original page number 17:
http://scholarsmine.mst.edu/thesis/pdf/Campbell_1937_09007dcc807ac958.pdf