One of those "looks good on paper" ideas that pretty much stays there.
Before the First World War the American rocket pioneer Robert Goddard
had the idea for making a train that would go from Boston to New York
in twelve minutes. It was simple enough. Just float the
train on magnets in a specially built tunnel with all the air pumped
out; that way all of the friction that slows a train down could be
eliminated and it would go like a rocket, pardon the pun.
Goddard's vacuum railway was never built, of
course, but it showed up several times over the past century. In
the 1920s, there was a proposal to build one from
New York to San Francisco. In the 1960s, it was considered by San Francisco as a mass transit system.
More recently, it
was considered for linking New York to London using a
tunnel under the Atlantic. And it formed the centrepiece for Gene
Roddenberry's ever-forgettable TV pilot
Genesis II, but
for all that, the idea never progressed any
further than proposals and bad movies.
Why? Money. The magnetic levitation
system is still being sorted out today and how to build and maintain a
tunnel hundreds or thousands of miles long, large enough to
accommodate a train, and that can maintain a vacuum at the same
time is daunting enough-- never mind building air-tight railway
carriages to run in it. Wait for the first run through of the
numbers and watch the investors flee out the door.
Pity, though. All that vacuum would make
flushing the toilets on the train a lot easier.