Jinman:
Without getting into a lengthy reply, I see from the position of the engine, I'd say that the engineer was in a braking mode as he crossed the bridge. All bridges are built on trunnions. The spans actually move laterally. If the spans were ridged, the effects of thermal expansion and contraction would cause the bridge to destroy itself. As a locomotive and its consist (cars) move across a span, the engineer cannot apply the brakes, as the weight of the locomotives and cars will cause the span to shift violently in the direction of the forward movement of the train. That is what it looks like happened here. If you cross and highway span, you will notice metal plates at each end. These plates are overlapped to cover the gap allowed for movement. In large spans, finger joints are used. If you sit in a stationary position on a bridge or span and there is vehicular traffic movement on the span, you can feel the movement relative to the flow of vehicles. Without that movement, the span would self-destruct.
If you take a 60 ton locomotive and a number of cars at say 30,000 pounds each and slam on the brakes on a span, the result would be what is in the picture.
