Looking at those images, and the way number 2 main is so wavy on the downhill side of the derailment, it makes you wonder if the high train forces against the track towards the inside of the curve were just too much and caused a track buckle underneath the train, and the 'forced inward' heat kink ultimately triggered the stringline. Weather that day was hot and humid, derailment happened after 3 pm during the heat of the day. My guess is the track was wanting to expand and the train forces gave Main 2 just enough of an excuse to kink under the train.
