Explain to me excess size. Each car has its own braking system and axles. I can understand not being able to initially accelerate or maintain speed if it is overloaded weight wise. I could also see a potential decouple somewhere if the train gets too long with too much weight without another engine or more attached every x cars or at the rear pushing. It needs to have distributed power to keep it all together. Each car has their own brakes, so while weight is added, they add braking capacity to keep it constant. So excess size caused derailment? That makes zero sense.