The video does not seem to show much if any, damage to the track on the bridge structure from overhead or in the horizontal photo. The bridge span girders seem unaffected a well. However the cat walk beside the rails is wiped out from the cars that slid over it when derailed.
That bridge, while laid out on an 1860's track plan, is much more modern than its original wood trusses. They were replaced with steel deck girder spans seen in the photos during the first half of the 20th Century.
From the position of other cars in that train to the left still on the rails, the empty covered hoppers that derailed likely string-lined when going through the reverse curve leading onto the bridge.
Both mid-19th century curves at that location are rather tight. Heavier or more cars at the rear of the train on the back curve could have allowed these long, empty and rather light weight cars to be pulled up and over the edge of the oppositely curved rails on the bridge.
Just my 2 cents worth of observation here, but a later report should determine the cause of that derailment.
S. Islander
