How old is that line? How many major earthquakes has it been through? Steel beams usually don't break like that without being very old and un-cared-for, and being overloaded.

Interesting, too, that the bridge fell out from under the tracks, but the opposite track remained connected. So the control system, if it relies on track circuits, wouldn't see the missing structure and would, in principle, go ahead and route trains over the track with nothing holding it up. That sounds like something PTC would miss, as well, if it occurs in a remote area where it isn't observed and reported.

This will be a good one to get periodic reports on. Something unusual happened to that structure, and we should learn what it is so other structures can be inspected for it.