They don't say: https://www.deseretnews.com/article/900005974/train-speeding-50-mph-over-limit-before-deadly-derailment.html but my guess is that it went off as the train derailed, not before. The article says that the sensors and controls for automatic braking were installed but not turned on. I suppose it is possible someone thought they were already turned on.
If we presume that to be true, then it would mean that the engineers didn't notice that the train was still doing 80MPH after passing BOTH SIGNS. Barring an equipment failure (actually there would have to be two 'mechanical/electric system' failures), the only thing that makes sense is that the engineer in charge WILLFULLY kept the speed at 80MPH to make it on time and he was told that he could make it safely doing so (even though no one had taken that train through that curve at 80mph in the training/test runs).
Add in to the mix that neither of the engineers reported any type of malfunction.