Automatic train stop systems are very rare here in the U.S. Outside of isolated transit lines, the only places you'll see them are the Northeast Corridor (Washington D.C. to Boston) and some of its branches, one or two lines across the west, and some parts of Southern California. However, these systems would not have prevented this accident. The systems in the U.S. and most everywhere else, would simply apply the brakes, usually by some form of electromagnetic transmission. The NYC subway does use tripcocks which physically activate a brake valve to apply the brakes and such a system
might have worked here, assuming each of the cars were equipped with the vehicle-side of the system, and the track was equipped with it. Something called Positive Train Control is mandated on all lines carrying passengers, and hazardous materials by 2015. This takes ATP to a whole new level and unfortunately, will likely reduce capacity on railroads.
However, I have three theories on how this happened. It was said that the cars got away from the locomotives they were coupled to. Normally, when a train separates, the brakes apply due to how the Westinghouse air brake system only relies on detecting a drop in pressure to apply, the train line doesn't apply the brakes directly. However, this means that in order to release the brakes, the locomotive air compressor(s) have to completely pressurize each reservoir on each car.
So based of this, I have three unofficial, personal theories neither of which may be accurate, but are certainly plausible.
Theory 1 is that the reservoirs were only partially pressurized if the engineer had made a lot of brake applications, without keeping the brakes released enough to recharge the system. The weight of the train cars, meant that the partial brake application wasn't enough to hold them.
Theory 2 is that the anglecock between the cars and locomotives was engaged, meaning the trainline wasn't intact, and the cars would not react at all to any pressure loss. This situation would be like the 1953 crash of the Pennsylvania Railroad's Federal into Washington Union Station:
https://en.wikipedia.org/wiki/1953_Pennsylvania_Railroad_train_wreck
Theory 3 is that the reservoirs were bled off. This is a normal procedure in "hump" classification yards so the cars can roll freely into the yard tracks after being uncoupled from the train. There are only a few situations outside of classification yards where this is supposed to be used, and it's hard to imagine why it would be needed on a long, line of the same type of train car (unit train).
