PTC was in effect and operating in the area, which would have controlled train overspeed. A review of the locomotive "black box" showed that train was operating a few MPH below the maximum allowed, 79mph in that area. The fact that cars derailed and overturned before reaching the switch means that if it was a track defect, it was prior to reaching the switch. Of course, it could have been a wheel defect also. We will have to wait for NTSB investigators to look at all possibilities.