[Eddyb (OP)]

Could a 5.6m diameter maglev train be used as the vacuum pump in a 6m diameter tunnel starting slowly at first and increasing the speed of each run until most of the air is pushed out the exhaust valve at the next 200 km passing station

In response to alancalverd I know about all the different things that did not work but I wonder if you could explain further about the inefficiency,

Do not forget that once there is only about 4t of air entering with every train once it is operational it would only be above atmospheric pressure in the last bit using the kinetic energy as it slows.

[alancalverd]

Yes.This is how pumps work, though it wouldn't be very efficient. Brunel's Atmospheric Railway operated on the reverse principle of a vacuum pump driving a piston through a long cylinder to haul the train, but it failed as rats ate the greased leather valves.

[watermaaan]

Very interesting!