> Come back and explain how to explain that interchange WITHOUT having a tunnel. If you listen for slightly longer when he talks about storing energy in the system so that it can run 24x7, it seems pretty obvious that whatever the system is, it has a lot of weight moving very fast.

Explain that exchange without it being a giant pneumatic tube like the Jetsons. If we take it at face value, he basically agreed that it's a pneumatic tube, but I'm pretty sure that's not really what he's thinking of, and you seem to agree, given that your proposal is absolutely not a "Jetsons tube".

I agree that storing energy without batteries likely means a lot of moving mass. That doesn't mean it's a big weight flying through a tunnel. It could also be a flywheel or something else. Or it could be that the "moving mass" is an incorrect guess and it's something chemical, but not in a traditional "battery" (e.g. a breakdown of water into component gases).

> We also know, because Elon Musk has said so, that, It is not an evacuated tunnel.

We also know that Elon Musk tends to propose (and build) practical solutions. It seems unlikely that he would be proposing what amounts to an evacuated tunnel with unnecessary complexity tacked on. If he's stating that it's not an evacuated tunnel, then it's presumably not just a semantic game.

> If it is going to be reasonably efficient, it can't have a lot of air in it.

So he's designed pointless complexity into the system just for the sake of saying it's not an evacuated tunnel. "It's not evacuated. It just doesn't have a lot of air in it."

It's be a lot simpler if he just proposed a traditional partially evacuated tunnel if that's what he wants.

> So those are the design constraints for what Elon is thinking of. What can you propose?

My lack of a proposal does not make yours feasible or sensible.

> Now why would you not want an evacuated tunnel? Well the common complaint about an evacuated tunnel is how hard it is to maintain the vacuum, and what to do if containment is breached. My proposal gives a way to maintain the vacuum (the plungers). Minor breaches of containment are not that big of a problem (the air that gets in is blown out again).

Your tunnel has all the same problems as an evacuated tunnel and more. A standard evacuated tunnel also has a way to maintain the vacuum. It's called a pump. This doesn't stop a breach from being problematic. You're ignoring the fact that pushing all that air out of the way again is hard, whether it's being done with a pump or a piston. If there's a breach, your "plungers" will run into huge pockets of air that have to be evacuated. This will slow them down, potentially causing collisions depending on how large the breach is. It will actually slow them much more than an evacuated tunnel, because they form a plug in the tunnel, meaning they need to displace most of the air instead of some of it.

Your plungers also have to form a tight seal in order for them to be effective. I'm not even sure that's possible. 70 tons of mass flying through a tunnel with a tiny gap all around. What happens when we hit that breach and this mass wobbles a bit? How big does the gap have to be to ensure safety? How big before the plunger isn't actually expelling gas anymore? And you admit that the friction the plungers encounter will make them hot enough to be deadly to humans. This is all wasted energy.

P.S. You also said this thing is going to be underwater. How on earth do you imagine than an underwater breech would be minor? And how is this air being expelled underwater anyway? Are you planning on building an exterior tunnel big enough to hold an air gap plus the transit tunnel (plus the return tunnel) all underwater? For a tenth of the cost of a bullet train?