On the subject of interterran transportation

What, my dear sir (or madam) is the quickest way through a circle or sphere? For your answer you have two answers, to go through its area or to go on its surface. Well, traveling at equal speeds, the answer is straight through it's area.

That is the fact that engineers use when constructing vehicles that travel within the earth. There are things as simple as tunnels by which to travel through the earth of a mountain, or machines as complex as the Topoträger (Mole vehicle). There are even vehicles that resemble trains, but they are cylindrical in shape (or what ever the shape of the tunnel is) and they fit quite tightly within the tunnel walls. They are used only to go back and fourth from one surface to another, either traveling a close diameter of the earth or traveling through a secant in the earth.