Last week, 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. The theoretical Hyperloop would consist of carlike capsules traveling at more than 700 mph through enclosed tubes. The capsules, which would contain about 28 passengers each, would ride on pockets of air, propelled by a linear induction motor. Some experts, however, have concerns about development cost, energy requirements, and technical feasibility. A Hyperloop, for example, could also be a tough fit in a region with an already built-up infrastructure, because of the need to locate the tubes and pylons.