This is getting old but I will say it one more time as clear as I can..
The engines were not derailed, nor did the wreck occur mid-train.
If you watch the vids closely, and read the posts, all is answered.
In the seconds just prior to the collision, its very unlikely the engineer had time to throw the train in the hole. After the impact, only he knows what transpired. From the videos its obvious the power stayed on the rails, but the flat car (2nd car behind power) was clearly on the ground.
Now at this point we don't know what is going on in the head end. Did the hogger hole it? Wouldn't matter anyway, the train went into emergency by default when the flat car broke loose and broke the air line. The head end is now free of the train, which has its brakes locked, but still 'accordians' into a pile until the brakes absorb the momentum of the rest of the train. I don't know how far from the pile up the head end got stopped, or if they found themselves stopped in the danger zone and moved further away ( knowing what was in those tanks just behind them) which would make sense.