A long-time science-fictional notion, a high-speed train inside
evacuated tunnel, is attracting support in the form of "we
to build it" projects. It is being called a "hyperloop". See the
for more details.
The main detail is that the degree to which the tunnel has its
removed by normal vacuum pumps, 99.9%, is still associated
significant air resistance when the speed of the train
1000 km/hr. I've seen some different ways to deal with it, like
compressing it at the front of the train and letting it loose
train (see second link, hah!).
This Idea is about something else. If a small amount of air
pressure in the tunnel is a problem, and cannot be removed
by normal vacuum pumps, then what about an abnormal vacuum
pump? Let the train be the pump!
Let us suppose that at 600 kph, the train stars encountering air
resistance such that you have to start worrying about
aerodynamics and stuff like that, to minimize the problem.
Suppose instead there was a normal vacuum pump at the front
the train? It could capture some of that air pressure building up
front of the train, and store it on-board.
When the train docks at a passenger terminal, there will be air-
lock connections for boarding and disembarking. The train
now extend an umbilical to the wall of the tunnel, and pump out
the stored air. Now the overall tunnel is slightly more
than it was before.
As all the trains in the tunnel do that simple thing, over and
again, the maximum speed in the tunnel can go up and up and
with air resistance no longer being a problem.