As I thought, excess speed. The turn is too tight, the trailing cars passed some of the cars ahead of them and being they are too light compared to the locomotive, they preferred to take the shorter path down side.
Akin to pulling a rope over ball. It will slip down.
https://finance.yahoo.com/news/engineer-derailed-amtrak-train-warned-210532152.html
An engineer on the derailed Amtrak train noted that the train was going too fast six seconds before the train derailed, according to the National Transportation Safety Board.
The accident occurred on Monday morning in Washington state, killing three people on the train and injuring dozens more.
The train was going 78 mph in a 30 mph zone.
Safety measures that could have potentially prevented the accident were not yet installed on the train.
