Gravity railroad
An interesting fact which i first encountered in Martin Gardner's note in the original "Annotated Alice" is that a train running on an absolutely straight track between two points (that is, one which cuts a chord of the Earth's surface), under the influence of gravity alone, will roll from one end to the other.
And a trip for any distance will take precisely the same time as a trip over any other distance, because, essentially, the train is the bob on a pendulum with a length equal to the Earth's radius.
Wouldn't be all that great for trips across town, but NYC to LA, it would be faster than Concorde.
And free, powered only by gravity.
(Well, once you spent the trillions to dig the tunnel, which would get pretty far down around Kansas City...)
