I can't tell exactly from the pictures, were those flats loaded with boards or timbers? Maybe one worked loose and caught the bridge structure.
It's too bad there wasn't also a boxcar full of bolts dumped on the ground; they would have had everything needed for repairs!
An old timer once told me that is how snow sheds were invented. A westbound freight train with construction supplies ran away on Donner Pass. It smashed through a stalled eastbound lumber train. The fastest way to clean up the mess and open the line was to stand all the boards on end.