Just happened to occur on a bridge and cars are scattered everywhere. Naw, nothing more than a mishap in the laboratory. Move along. Nothing to see.
The train left the tracks out-of-frame off to the lower left of the photo.
It appears that the front unit stayed almost on course, and the remainder of the train "whiplashed" out toward the adjacent rail line.
The piece of car at mid-upper right (at the end of the row of trees) appears to be the front end. If the initiation source was onboard, I'd look for evidence of the source in the engine unit.
The burning debris at the lower left corner of the bridge should contain some clues...
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It must have been nearly empty of passengers -- otherwise, the casualty toll would have been much higher, IMO...
Very interesting picture. Appears the motivepower, at lower left in the pic, left the tracks well before the bridge. I would imagine that the rest of the train did as well. Any more pics of the bridge itself?