Why are you inventing constraints that don't exist?Elon's pauses make it clear to me, and presumably to you, that he was checking what she said versus what he was thinking, and finding that, to a layman, the two would seem similar. Even if underlying technologies are different.His wording suggested strongly to me that his solution was a tube. Did it not suggest that to you?No matter how many times you say "pointless complexity", that doesn't mean that there really is pointless complexity. (And we're disagreed on the complexity.)I'm quite aware of how hard it is to push air out of the way. A quick comparison of the mass of the plunger and the mass of the air says that a bubble of air will not actually slow the plunger down much. Being in air constantly would, but a bubble of fixed size wouldn't. (You'd have to think about what effects it might have on passengers inside.)There is absolutely no need for a tight seal, as long as the flaps open and shut at the right time. You can stand a long ways away from a moving truck and still feel the wind pushing away. And the closer that it gets to Mach 1, the farther away you can feel that gust.The friction losses are hard for me to estimate. The upper limit on the temperature of the plungers is caused by the heating of air that is suddenly compressed to 1 atmosphere, and the friction from air moving past at hundreds of miles per hour. However if only a little bit of air is involved, the heating might happen at reasonable rates. Without parameters that I don't have and a fluid dynamics simulation that I am not prepared to do, I can't tell. That said, I can tell you that the rate of heating per volume (and therefore the ease of dealing with it) goes down as you make the plungers larger, up as you speed up, and down as you reduce air pressure. So if you have little enough air and large enough objects moving through it, the heat generation gets more and more reasonable.And about your PS, I'm trying to guess at how Elon gets around right of ways. Underwater is not part of my basic idea at all.