The train was too long for the track he arrived on, or he overran his stopping point. Either way, the train was parted, and he pulled ahead, a switch was thrown behind, and he proceeded to back the front half of his train into an empty arrival track adjacent to the one he arrived on.

Now I'm no professional railroader, but I've seen many a backup move in my time, and in my opinion, the hogger showed a bit of enthusiasm in his reverse move. That, coupled with the bad condition of the yard tracks, probably contributed to the scene you see here. I had just walked over from the north side to the south side of the Laramie Avenue bridge as the reverse move began, complaining to myself that I should have started shooting on this side instead of the other side, where the passenger triple track main was located. As I arrived on this side (the south side), the bridge began to shake, not too alarming as heavy trucks crossing the bridge would also cause the bridge to shake. But there was considerable noise very unlike a heavy truck, and I soon found myself witnessing a derailment happening literally under my feet. My impulse was to run off the bridge, as there was the possibility of errant freight cars hitting the bridge support, and I would have joined in the derailment. Just as I got my feet into Run 8, there was the PSSSHHH sound of the emergency brakes setting up, and everything came to a halt in a cloud of dust. Behind me, I heard the turbos settling down to idle. I'm not sure if the hogger set the brakes or if an air hose parted, but I do recall the turbo sound still whining in a power setting after everything came to a stop, then winding down.