A gravity train is a theoretical mode of
transportation intended to go between two points on a sphere. It follows a
straight line through the center of the sphere. If you've seen the remake of
Total Recall, you might remember that a gravity train was a major plot point of
the movie.

On earth, a gravity train would take 42
seconds to to go from any two points on the planet. This would be the case if
it was a perfect sphere and of uniform density. There are a number of reasons
why a gravity train is a practical impossibility.

For starters, the pressure that a train
would be put under through the center of the Earth is extremely high. There is
no known material that could withstand that kind of pressure. Extreme heat
would also be a problem. Finally, there tunnel would have to be completely
airless, because air friction would make the train unusable.