Imagine a tug that was runaway... the monorail isn't the only powered thing on the track
The tugs have rolled downhill into things.. and imagine if a tug at power ran into a stopped train because the tug's controls failed or were mishandled.
I'm coming into this late (and maybe there is more in the thread yet..) but based on the images so far.. I'd be more suspicious of something like that... the tug slamming into the train. The trains aren't ran in reverse under normal use anymore.. There wasn't anyone in the cab it seems.. so the train wasn't likely moving in that direction under power. And if it was moving that direction, due to automation testing... I find that highly unlikely given the sheer insanity of running the two vehicles at each other. No way they are that dumb.
Maybe it's something like the train was out for automation testing... stopped.. and the tug was sent out.. then the tug screwed up and slammed into the train. That would make all the stories we heard so far be compatible with each other..