There's a diamond (tracks crossing and not merging)there. The diamond is directly under the highway overpass. Apparently the BNSF train was crossing the diamond and the locomotives and most of the cars had crossed. This should have put a restricted signal on the UP track for the west bound UP train that should not have cleared up until the BNSF train was clear of the diamond interlocking.
Here's the spot (assuming it loads). UP is the E-W track and BNSF is the N-S track. You can see the signal towers if you zoom in, and the report at another source said the BNSF train was hit about 8 cars behind the locomotives meaning it had already entered the interlocking and that should have tossed a 'red' on the UP signals. What really happened is not known at this time, but either there was a signal failure or the UP train blew through a red light...
Oops — you beat me to it, NoCmpromiz!
Thanks, never saw that before.