Imagine that you are travelling from New York to Seattle via an underground tunnel.
A tunnel via the center of the earth would be 0 gravity much similar to space.
The space does not have any gravity.
Similarly the gravitational force of attraction varies as the distance between the center of the Earth and the object.
So when an object approaches the center of the Earth its weight becomes zero.
According to classical laws of motion the object will travel in Simple Harmonic Motion and will reach Seattle from New York in about 42 mins.
Can you actually travel inside the Earths Surface.
What would be the limitations?.
The limitations would be the heat inside the Earth.
Another limitation would be the water present inside the Earths Surface.
Both these would prevent constructing a network of tunnels inside the surface.
Also it may take ages before a tunnel system of transport can be publicly constructed.
The only advantage of travelling via a shortest path inside the Earth is the absence of gravity and also the distance would be shorter inside the Earth rather than on the surface.
Also it may not be possible to dig the Earth due to environmental concerns as it would cause damage to our natural geological environment.
But the surface of the moon presents an ideal scenario for constructing a tunnel because of the absence of water inside the surface of the Moon.
This will create a perfect vacuum for any material object to travel inside.