Life-support apparatus is a well developed field. Space stations in orbit allow astronauts to breathe for several months, even though the capsule is in a near vacuum. Submarines have been around for over a hundred years, modern subs can stay submerged for more than a month. The systems used in Evacuated Tube Transport will be much less demanding. Only 2-3 hours (plus reserves) of life-support is needed.
How can people breathe?
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