We recently covered what was known about Elon Musk's mysterious Hyperloop; he describes it as a "cross between a Concorde and a railgun and an air hockey table", and says that he will publish his design without patents, under open source license by August 12 (soon!).
@John_Gardi your guess is the closest I've seen anyone guess so far. Pod diameter probably around 2m— Elon Musk (@elonmusk) July 15, 2013
So far, nobody except Musk knows exactly what it is, but John Gardi (who we mentioned in our original story) used what was known and took some guesses, creating a design that Musk himself, on Twitter, has called "the best guess so far".
Gardi decided to elaborate on his ideas in a fascinating article. I recommend you go read the whole thing.