The reserve tanks on each individual car still closes and holds the brakes. Which is why the train did not start rolling for more than an hour.
The tanks are designed to give the operators time to set the hand breaks.
But the train was left abandoned, and procedure was not followed setting sufficient hand breaks prior to leaving unattended.
That makes no sense. Air breaks are FULL ON when there is NO air pressure in the lines.
The brakes were either not set initially or were released prior to the train rolling.
It was not a systemic failure it was either operator error or sabotage. Probably the latter judging by the fire.
Someone released the brakes.