If we're entertaining random speculation, the first thought in my head was something involving the new engines.FBI at this point is saying it doesn't appear to be a act of terrorism. IF the train was doing 106 Why did it go that fast and why didn't the systems in place stop it? maybe a medical issue with the engineer and he slumped over on the throttle?
There were probably multiple factors involved so it's important to avoid too much rampant speculation.
That being said one really has to hope it's not the new engines.