Not sure where your news story comes from, but it does answer many questions.
It would appear that an insufficient amount of handbrakes to hold the train were applied when the train was left unattended. If more were applied and then taken off by somebody else later, that is for investigators to find out.
The simple fact of the engine being shut down would be irrelevant. This can be compared to the brakes in your car. The car needs to be running for the brakes to work easily, but the parking brake will work whether the car is running or not. If there were more handbrakes, all the train brakes could be released and it should not have been able to move.
The brakes on the train don't need air pressure to be applied. They actually work in reverse. The train brakes need air pressure to release and charge the brake system, and then it is a reduction in pressure that applies them.
As equipment gets older, it will develop leaks in some of the reservoirs on each separate train car. As these air reservoirs slowly leak air into the trainline, it starts a chain reaction of each car pumping air into the system. (remember this is without the engines running) As 1 car releases, it starts pumping air into the trainline, which in turn causes the cars beside it to also start pumping more air into the trainline. Eventually all air brakes release.
If there were more handbrakes applied when this happened, the train still would not start rolling once all the air brakes released. If the engineer had a sufficient amount of handbrakes and then had tried to move the train prior to leaving, he would have known it was safe to leave.
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