Exactly one million years ago, when I was a student at the University of Illinois institute of Aviation, there was a motley crewe who came by on a regular freight run in the middle of the night in a flying boxcar. In exchange for a couple of beers (when not flying of course😎
one could bum a ride in a real turbine twin. I availed myself of this opportunity and had the experience of having the crew turn off my O2 at 21K. (Just for giggles). When they turned it back on, it was like Dorothy getting to OZ.
Louder than hell, but a lovely freight dog