Lt Col David Och's thesis paper that Steve R mentions above can be found here:
https://apps.dtic.mil/dtic/tr/fulltext/u2/1015778.pdf
On pdf page 97, (in a section where he is discussing the 397th BG, it says:
"Between 16 and 23 July, the group attacked four bridges and one railroad
embankment ranging from the Paris area to the Loire. Although some missions
brought little success, most achieved probable damage to the structures. The
group scored direct hits with their 19 July attack on the rail bridge at La
Possonniere. In addition to destroying five spans on the bridge, they reported
hitting unlucky freight cars transiting during their attack."
On this link:
http://www.b26.com/marauderman/sterling_hoch.htm
which contains information provided by Sterling P Hoch (397th BG, 596BS)
It says this on the 19th July mission:
"Up at 4:00AM – mission scrubbed just before take-off. Another long mission in evening – milk run. (La Possonniere RR bridge)."
While not available on-line, the B-26 Marauder Archives at the University of Akron do contain Strike Photos of the attack. They are in the Henry C Beck, Jr collection.