To build a hyperloop transportation system, engineers must create a near-vacuum in miles-long tubes, deploy powerful magnets to suspend capsules on a sliver of air and propel passengers at nearly the speed of sound without making them sick.

Yet that may be the easy part. Backers of two companies racing to build the futuristic transport system insist it is technologically possible. The bigger challenge may be proving it commercially viable, sidestepping the fate of Concorde, the supersonic jet that airlines retired in 2003 because of its high costs.