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 floating vacuum tunnel under the Atlantic. And it formed the centrepiece for Gene Roddenberry's ever-
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-
Pity, though. All that vacuum would make flushing the toilets on the train a lot easier.