Right, but if the recorders were used on general-purpose trains, they might be able to, with proper software, detect rail problems which develop between inspections.
It is very doubtful that applying the emergency brakes caused the derailment. On a normal passenger train(say less than 20 cars) the risk is near zero. This was a longer train(39 cars, about 3200' long), but the real risk are for long freight trains(100-160+ cars cars, 1-2 miles long) with different types of couplers and more of them which would increase the slack action. The real key is the weight, with heavy cars slamming into light cars. But there really isn't much weight difference between the rear auto carriers and the passenger cars, so I doubt it made much of a difference here. Even with freights, the risk is low. I can't tell you how many times I've been on long, heavy trains that went into emergency, but not once did they derail. But that doesn't mean we don't sweat a little each time it happens(especially a conductor, because he then has to walk the entire length of the train(and likely back, could be a 4 mile nature walk on uneven rock ballast the size of grapefruit) to inspect for damage or problems. No fun on a cold rainy night of lightning on a steep grade, where he'll also have to set and later release a dozen or two hand brakes, and if he does 'em too loose, it'll roll away!)
Plus there is a device on the rear of each train that can apply the emergency brakes from the rear if the engineer hits the button(and on some engines this is automatic). So if the engineer puts the train in emergency and also hits the rear switch, it will apply for both ends of the train and substantially reduce the amount of run-in that would take place. Train brakes are based on air, and the engine has to keep pumping air to keep them released. If the air drops, they will begin to set. An engineer normally applies the brakes by reducing the air pressure by a certain # of lbs./sq. inch, a little for a slight reduction, more for greater. What the emergency brake does is evacuate all the air out of the brake line, for a full application. But each car's brakes are also designed to allow the wheel to continue rolling and not lock up. So it still takes a ways to stop from 56mph(it isn't like an automobile) and the passengers often don't even feel the difference of an emergency application. I doubt the noise some said they heard was the brakes, but rather the effects of the accident unfolding. Bottom line, it is always better to get the speed down when approaching a defect, especially for a passenger train.
Am hearing more talk of a sun kink, but that's unusual for the weather that day. 81 degrees is not real hot, the low earlier was in the low 60's, so that not an extreme range of temperature change in a short time. But perhaps some combination of direct sunlight and substandard maintenance. Mainline rail these days is usually a long continuous strip, as opposed to the old 39' sections historically used. The advantage of CWR(Continuously Welded Rail) is that substantially reduces these joints, the most likely place for a failure(that's why you rarely here the rhythmic clickety-clack sound anymore, which was the wheel hitting the joints every 39'). The disadvantage is there is much less room and outlets for expansion due to temperature change that the joints would have provided. So if some of the anchoring devices that hold the rails to the ties fail under stress, a kink can result. Might not have been enough to affect a heavy slow coal train, but trip up the passenger train. Perhaps the coal train triggered the failure as it passed over it. Another possibility is a broken rail, where it actually separates. But those are much harder to spot at high speed. And there are still rail joints at certain locations, so one rail could have broken loose at a joint and become misaligned enough to be visible.
As to inspections, the in-depth analysis to detect hidden rail flaws(some x-ray type computer device) happens on a regular basis, but I don't think it is required mored than 2 or 4 times a year. That's why the visual inspection is so critical. Nothing is failsafe, but often times it is a judgement call, walk most any mainline and you can find minor defects such as loose spikes, fasteners, bolts, etc. However one here or there isn't that critical, but when several go in a small area, then there can be problems.
Hard to say yet what happened. Seems pretty clear that there was a track defect. Whether it was act of God, sabotage, or due to poor maintenance isn't clear. And there is also the chance of some type of equipment failure on the cars that was the actual trigger, preventing the train from riding out the track defect.