Actually, the speed limit was 79mph prior to the curve and the train was doing 81mph. SO.... regardless to the approaching curve, the train was already doing OVER the speed limit ANYWHERE on that track.
WHY? Because they were 30 mins late in even leaving the station and were behind schedule (the schedule that claimed the new route would save around 15 minutes versus the old route).
black box showed that the brakes were being applied just a few seconds before the derailment.