I can tell you this, the train engineer, will be told to submit to a drug and alcohol test immediately.
As to why the train didn't slow down or stop?
I don't know.
The dynamic breaks, should have slowed the train.
I know in the cab, the engineer has to press a button to acknowledge that he is awake, every few minutes. If he doesn't, the train is supposed to slow down on it's own.
Maybe it was a malfunction that didn't allow for the breaks to engage.
Maybe the engiineer wanted to be a speed-demon.
It'll be intersting to see what the investigation reveals.