I read this idea in the 70s in Newsweek. It was probably old even then. Imagine a straight-line tunnel between Portland and New York. In the first half of the trip, the subway goes downhill, powered by gravity to a thousand miles per hour. In the second half, it slows going uphill, away from Earth's center. Because of friction, it needs a motor to boost it the last miles. It won't work. Musk's degrees are in engineering and economics (hence his rocket emphasis on reusability), but he's also a salesman.