No cause is known yet, but based on the videos, what’s the most likely reason for crashes? Bad tracks? Some human error resulting in collision?

I don't want to speculate on this crash but my mental model for these things is that there's always a handful of factors that all align and converge to create an accident. Some factors are deep-rooted - and point to decisions made years ago - sometimes related to company culture. Theres always an element of operator error: someone ignored something due to inattention or sleepiness.

What's the befit of speculating at this point? Let the investigators investigate, and hopefully some lessons will be learned.