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I haven't done any work on hyperloops. But expansion seems to be a problem already solved by industry a long time ago: https://en.wikipedia.org/wiki/Metal_expansion_joint

Sounds promising:

- Resistance to high and very high pressures

- Large movement absorption

- Early leak indication (in case of damage) via standard check hole

- Complete burst resistance

- Possibility of permanent leak monitoring in critical media

Metal expansion joints can't hold a vacuum.