The TSA was born out of 9/11 - prior to 9/11 the assumption was “we’re been taken to Cuba” and so no one fought back until it was sadly too late.

In the post-9/11 world people aren’t going to just silently wait for rescue, and the core threat (the ability to enter the cockpit) has been stopped.

Hijacking a plane to go somewhere (at least in US airspace) isn’t going to work.

So any attack will be terrorism. Assuming something planned (as 9/11 was) you can “easily” kill a lot of people with a train - as numerous accidental derailments have demonstrated.