Looks like a roller bearing on one of the trucks failed. It may have held on for a while (hence, the video with the sparks/fire some miles back), and then "let go" quickly. It wasn't on a tank car, either, but on a covered hopper.
I haven't read anywhere as to WHERE the failure occurred in the train -- toward the head end, middle, toward the rear, etc.
I read earlier that the train had something like 141 loads and a few empties, getting close to 150 cars. With car lengths in a general freight train running 50-70 feet long, that's a mile-and-a-half +/-.
On the engine, you just can't see back that far. In most places you're lucky if you can see 30-40 cars back, if that far.
But these days, with their new-fangled concept of "precision scheduled railroading", they double-up two trains into one monster train, and run that (often with an engine "in the middle" and perhaps another on the rear, all controlled from the head unit). Fewer crews, less fuel consumed, more $$$ per train.
This can make handling more difficult, as well.
It will be interesting to see how the FRA addresses this. In the case of an accident with serious side effects (such as this one with the release of toxic chemicals, the full extent of which isn't yet known) there's going to be pressure to "do something". Usually this means some new kind of operating rule or federal regulation.
But not sure what they can do here. If the engineer wasn't doing anything he shouldn't have been doing (over the speed limit, etc), and the trackside defect detector didn't sound off until immediately before the train derailed (sounds like that's what happened), they probably can't pin this on the crew (which is the easy way out insofar as the company and feds are concerned).
They might go after the car department for not inspecting the train properly, but if the defect had not yet developed at the time the train left its initial starting point or passed the last point where there was "human inspection", can't go after them, either.
I'm thinking the feds may order that cars containing particularly hazardous materials travel in shorter, dedicated "haz/mat" trains, with lower speeds and frequent "human" inspections.
[Hey, did I ever tell ya about the time I had a special tank train of 40 cars of sulfuric acid, with NO BRAKES? No "properly working brakes", anyway. Got it from Pt. Jervis (NY/NJ/PA border) to Oak Island (outside Newark) without "using the air", at all!]
