There would be no dynamic brake as the brake pipe was at zero due to automatic handle in emergency position.
You need brake pipe pressure for dynamic to work.
Some serious questions to be asked in the investigation.
The previous crew ran out of hours and were waiting at the top of a heavy grade for relief. The train had been sitting on an emergency application for 2 hours when relief crew got on train.
Wagons brakes may have started releasing in that time and in such freezing conditions there would be air leaks for sure from brake pipes and equipment increasing chances of brakes releasing.
Why did they go to an emergency brake application ?
Having sat so long why didn’t crew apply handbrakes to wagons?
They could have sat on a full service brake application with no issues.
Did relief crew just release brakes and hope they would get their brake pipe recharged before needing to apply brakes or use dynamic brakes ?
Did they expect the locos independent brakes to hold train on grade while brakes recharged ?
Tragically the crew is dead and can’t explain what the exact circumstances were.