Imagine strapping into a car-sized capsule and hurtling through a tube at more than 700 mph ? not for the thrill of it, but to get where you need to go.On Monday, billionaire entrepreneur Elon Musk unveiled a transportation concept that he said could whisk passengers the nearly 400 miles between Los Angeles and San Francisco in 30 minutes ? half the time it takes an airplane.If it?s ever built.His ?Hyperloop? ...