About four hundred years ago– sometime in the latter half of the 17th century– Isaac Newton received a letter from the brilliant British scientist and inventor Robert Hooke. In this letter, Hooke outlined the mathematics governing how objects might fall if dropped through hypothetical tunnels drilled through the Earth at varying angles. Though it seems that Hooke was mostly interested in the physics of the thought experiment, an improbable yet intriguing idea fell out of the data: a dizzyingly fast transportation system.
Hooke’s calculations showed that if the technology could be developed to bore such holes through the Earth, a vehicle with sufficiently reduced friction could use such a tunnel to travel to another point anywhere on the on Earth within three quarters of an hour, regardless of distance. Even more amazingly, the vehicle would require negligible fuel. The concept is known as the Gravity Train, and though it seems inconceivably difficult to construct, it has received some serious scientific attention and research in the intervening centuries.
They’re literally straight line tunnels from one point on the Earth to another. Jump in and gravity does the rest. Were it possible to design a system with no friction, you’d arrive at the exit as your speed reached zero – at which point, presumably, something would have to stop you falling back in. No matter how large the vehicle or how great the distance, the travel time would be about 42 minutes (approximate because the Earth isn’t a perfect sphere). The article has much more detail, as well as the extreme technological hurdles that would have to be overcome to build one. I’m surprised this concept hasn’t turned up in any sci-fi I’ve read, although maybe it has without me registering it as a plausible device.