concordgirl wrote: I'm curious about how [NTSB] deal with issues for which there will be no definitive evidence one way or another. How thoroughly do they look into less likely scenarios like signal malfunctions?
The NTSB are nothing if not thorough, they're experts when it come to looking at an accident. They're technicians and they don't hesitate -- based on the reports I've read -- to admit when they could not make a conclusive finding.
I believe the NTSB is largely responsible for the locomotive and signal box data recorders we have today. In earlier times they lobbied hard for them. (Something the railroads resisted at first on cost grounds.)
Btw, I have read NTSB reports where malfunctioning signals were involved. One in Texas was due to a maintainer's error. Another involved a freight train being unable to stop in time to avoid hitting another train at a junction. The student engineer claimed he never saw the distant signal -- that it must've been dark -- and by the time he realized he'd passed it (when he saw the home signal displaying 'Stop') there was not enough distance to stop the train short of the collision. There were
other factors, but NTSB witnessed the signal go 'dark' and discovered there was a defect, not just with that signal, but that entire model of signals. There was a case in Pennsy involving a Penn Central freight passing a stop signal that, when the sun hit it a certain way, did appear to be yellow when it was actually red. I don't want to say more -- I don't want to be reprimanded by the mod for going off-topic -- but all of these were documented by NTSB. I have the reports.
But when you think of the thousands of trains that operate everyday, and the hundreds of thousands if not millions of correct signal indications they receive, the systems are very reliable. I too seriously doubt malfunctioning signals played a role at Chatworth. But you hope the investigators keep a open mind so on those rare rare occasions when it does happen they will uncover it.