BBC reporting from a passenger that there was an early announcement that the front four coaches heard, something along the words of "Sorry we've stopped, I'll get back when I have more information".
I guess a driver might initially be unaware of a derailment further down the train. That must raise issues with whether to press the "stop all trains" button?
I'm not suggesting anything was done wrong. Presumably, if the emergency brakes come on, you might just think the air pipes have simply leaked?