Hi all-----The University of Louisville has the official L&N archives, with thousands of L&N plans, photos, and documents. Even though the Big 4 bridge was not an L&N structure, given its proximity and relationship to the L&N right there on the riverfront of Louisville, there is a good likelihood that many photos and maybe a copy of the plans would be in that archive. I lived in Louisville 1991-1994, and the bridges there are something to behold. The former Big 4 yard across the river became (as of my last knowledge in 1994) part of the Louisville and Indiana Railroad.
Best to all,