While I confess that I don't know the mechanics of train air brakes, I thought that a positive pressure released the brakes and that if there were no pressure (as would be the case if the engine's compressor was NOT running), the brakes would be locked.
I'm assuming that this was a fail-safe system designed to stop cars from rolling if they got disconnected from the rest of the train.
It’ll be interesting to see where the locomotive train air/brake brake lever allegedly was set after the locos went through town and stopped.
I still think it's questionable to leave ~$8,000,000 of hazardous material next to the side of a road without adult supervision. It should be in a more secure place when parked at night.
Just where this disconnection happened and at what speed could be a big part of the investigation.
If the train came through town at 40 to 50 mph and the disconnection happened right in town, the pile up could easily happen before the tail part of the train stopped.
They (including maybe the MMA engineer - who was already in town) pulled some 13 intact tank cars off the rear of the train and away from the fire at some point shortly after the event.