“Short of figuring out real teleportation, which would of course be awesome, the only option for superfast travel is to build a tube over or under the ground that contains a special environment,” he wrote. Capsules could depart as often as every 30 seconds and could also carry cars. Passengers seated in cabins inside the capsules would experience slightly more than the force of gravity, more like on a plane than on a roller coaster, he said.