[TXnMA]

Thanks for the OHI! Here it is -- post-processed to enhance visibility of soil disturbance:

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...

[TXnMA]

If it was, indeed, traveling at TGV speeds, that would account for the front unit traveling almost airborne and almost in a straight line from the point of rail departure...