No caboose to see the air release.... No one to check, or to roll by the train, and catch the caboose anymore... Did they walk the train? Did they drive the train?

Turn an angle cock, pull a pin, and no one notices... It has happened before, with worse consequences... Vandals pulling pins and turning angle cocks is not a new problem. Knowing enough to do it with the train line charged...? Definitely worth looking at something like that for sabotage. And, given some the nutjobs on the fringes out there, anything is possible these days....

I'll wait on the NTSB report, and any subsequent FBI investigation, though...

Failure to properly inspect the train they had relieved-hurry up-let's go.... might just be one reason there might be an "investigation" of the conductor under way... I've seen that one, too... And, this is the Union's spin for their guy, and it's what they're supposed to do for your dues money... But, until I see the final NTSB report, or the FBI issues a statement confirming it, and speaking as someone who has seen the union defend both the "guilty" as well as the "innocent", as they are both entitled to effective representation, I'd take that perspective with a grain of salt....