If there was an obstruction in the train line, the DPU unit would've sensed it and wouldn't have passed an air test. Doesn't matter now as we know all the power was on the head end.
As for FRED, if the engineer wasn't paying attention to the head end device to see that the air went to the rear, that would've just added to the mess.
Time will tell what happened at the yard when they did the pick up of the cars. If they did a proper air test on the cars and a set & release of the entire train when they got it all together. Fuck the managers, they can bitch and moan about their departure times all they want. I'm not leaving a yard if the train has issues, especially if those issues are with the air!
Working out of L.A. and having to deal with Cajon Pass or Beaumont, I know what's going to happen if things go wrong on either hill. I know people that have died on both and the number one reason is losing their air. When those heavy trains have issues, it's better to stop and fix it then die because a manager wants his bonus.