An update on this train derailment, and reconditioning of the foot bridge:

https://www.winchesterstar.com/repor...7df18fa22.html

It sounds like the train was stopped with the brakes on, and the engineer went "full speed ahead" at the engine, and pulled the line tight (as you might when pulling a curve out of a string) and it yanked the cars off the track. The news article claims the foot bridge is expected to be back in service in late July.