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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.

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