I don't believe that it's Big Bayou Canot. I've heard stories of this indecent happening but, I cannot find any records to validate them. It would explain what appears to be a support for a swing.
Are you sure you're not thinking of this bridge.
http://bridgehunter.com/al/mobile/csxt-big-bayou-canot/
https://www.youtube.com/watch?v=sJMOghxT0AE&feature=youtu.be
It would appear this bridge was originally a swing span, the three southern supports being part of the original swing (center support of the three being the pivot). Apparently, in the 70's or 80's, part of a load either hit the bridge or the span was weak and the train derailed, fell off the swing span and into the river. The accident rendered the swing scrap and a new deck was built over the old swing supports and a new lift installed (the current lift in place). I cannot find any information on this, any insight would be appreciated.
Was definitely the Big Bayou Canot in 1993.
Amtrak's worst accident [47 lives lost, I believe].