Even with a slide fence or similar detector and PTC, this wreck could still have happened if the rockslide occurred shortly before the train arrived. We don't, of course, know exactly when it happened. PTC might have detected it (with slide fences) in real time a bit sooner than the engineer, if it was around a corner and beyond visual range (likely, in Vermont), and slowed the train a bit more before hitting it. But the rocks would have needed to fall far enough in advance of the train to allow a full stop for PTC to prevent the wreck. All of this is asking a bit much of Murphy.