Another Derailment at Santa Fe Jct Caught on Camera - Trains Magazine

That comprehensive video is extremely interesting, both with the close-ups and the inclusion of the head end of the train before and during the derailment.   While the bang could have been an explosion unrelated to the train, I conclude that it was caused by the train, and originated by the gray covered hopper that was directly in front of the camera when the bang occurred.  Interestingly too, is the appearance of debris falling off of the car at the instant of the bang. 

You can see a few chunks of something maybe a few inches in size, falling from the car on its side opposite the camera.  You can see them through the three large open spaces under the carbody, between the hopper bins.  Each of the three spaces reveals falling pieces of debris simultaneously during the instant of the big bang. 

The perfect time correlation between the bang and the falling debris is strong evidence that the two events are related both originating from some type stress release within the structure of the hopper car.  While it does sound like a hard coupling impact, there is no indication that it came from slack run-in. 

The video opens with the passage of the three engines and the first two cars, and then that sequence ends and another begins with the passing cars, including the bang and the sound of the derailment and dynamiting brakes just after the derailment begins.   Since those are two independent video sequences, it does not indicate how many cars and video time there was between the two sequences. 

The reason I mention this is because the train seems to have increased its speed in the second sequence compared to the first sequence.  The second sequence is when the bang occurs, and there, the train seems to be moving a few MPH faster than in the first sequence. 

The fact that no slack was heard running in before or during the derailment suggests that the slack was bunched during the entire video, or at least the part begining with the second sequence.  Indeed the derailment itself seems be caused by excessive compression/draft force, which could only occur with slack bunched.

So if the train accelerated during the video, it follows that the acceleration was caused by power shoving on the hind end.  The first evidence of the excess draft force was the bang associated with the covered hopper.  The second evidence was the derailment.