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Why does everyone expect this "tube" to be filled with air? A liquid is more interesting and it also stores a lot more energy when it moves ...

If you can find a liquid with low enough friction to be feasible, I'm all ears.

The only ones that I'm aware of are thin gases or helium-II. The latter is rather impractical to use for a great many reasons. But I'd be interested if anyone has other suggestions.

Are you talking about friction between carriage and liquid or walls and (moving) liquid?

The liquid will move at the desired travel velocity, so I believe the latter will be important.

Air is a fluid, too. It's also got a lower viscosity than say, water, so it will create less resistance.

Saying it stores a lot more energy when it moves is like saying that we should make our train cars heavier so that we can recover more energy by regenerative braking.

Because the resistance of any economically feasible liquid is probably higher than that of air. The energy needed to overcome that friction doesn't drop just because the water carries more momentum.

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