After removing the air and putting on special equipment, you could soar freefall through the tunnel.
To think as a child, you can dig a hole to get to the other side of the Earth is very funny and seems very simple. Simply jumping in the tunnel and you let gravity lead you to your destination. If reality would allow such a "journey", however, things would not be as simple as they seem at first glance.
Firstly because gravity will accelerate the movement of your body towards the center of the Earth, but once past the "middle earth", gravity would start to work against you.
That's how long would it take to get to the other side of the Earth. You must first create a vacuum by removing the air from the tunnel, otherwise friction, as an air mass that resists permanent would you slow down too much. In addition, the air pressure with how close the center would not crush the human body can resist.
Further, you need to wear an astronaut suit that would protect you from the intense heat of "core" and you would provide the necessary oxygen. After removing the air and putting on special equipment you could soar freefall through the tunnel. If gravity is constant, you would need about 19 minutes to reach the Earth's center. That means you plunge 25,000 kilometers per hour and you feel completely without weight, like an astronaut in space.
As you move the center of the Earth, the speed would slow down because gravity would push you against them, so that, once the other end of the world, catch normal rhythm again. In total, the trip takes about 38 minutes, which sure beats a flight of 40 hours from Santiago, Chile, to Xian, China.