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So, in this scenario, people are living in an Earth-sized planet, with an atmosphere. They travel via ultrafast train that carries them to the solid core of the planet, where they then transfer to other lines that take them back to the surface. However, an issue is that in the deeper parts of the tunnel, air gets denser and denser as there's more air pushing down, and I'd prefer to not have hundreds of airlocks. How else might I simply solve this?

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  • $\begingroup$ The pressure would massive. The energy to pull the vacuum would be massive. $\endgroup$ – paparazzo Jan 1 '17 at 22:59
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    $\begingroup$ You might want to reconsider your tunnel-to-the-core concept. Most of the interior of an Earth-like planet is REALLY hot. $\endgroup$ – John Feltz Jan 1 '17 at 23:13
  • $\begingroup$ As Paparazzi and John Feltz said, the pressure and temperature at the core of a rocky Earth-size planet are overwhelming, making any kind of material to have very strange properties. AFAIK there is no way to have a hole in a planet's core. Air getting denser and denser is by far the least of the problems. $\endgroup$ – AlexP Jan 2 '17 at 0:54
  • $\begingroup$ As others have noted - if they're tunneling through immense heat and pressure that melts metal - and the densest materials on Earth - while defying the entire planet's gravity for thousands of miles of sustained transport - air pressure is the least of their concerns. $\endgroup$ – Zxyrra Jan 2 '17 at 5:28
  • $\begingroup$ A solid core and an atmosphere are generally not compatible. $\endgroup$ – Xandar The Zenon Jan 2 '17 at 21:42
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Just have your ultrafast trains travel in a vacuum. When a train docks at a station, airtight connections will allow passengers to get on or off. When the train is ready to go, the connections will be closed and withdraw, leaving the train in a vacuum tube.

This will also allow your trains to travel much faster - with no air in the tubes, there will be very little friction, and you don't have to worry about the shockwaves you get when you approach the speed of sound.

Now you only have to worry about keeping the air in the central station fresh. However, seeing as you have the technology to send trains to the center of the planet, keeping air fresh should be a fairly simple task.

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    $\begingroup$ Only way to do it. $\endgroup$ – Karl Jan 1 '17 at 23:36

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