Maglev (magnetic levitation) trains can save energy, but the "savings" depend heavily on the speed at which they operate and the technology used. Because maglev trains "float" on a magnetic field, they eliminate rolling resistance—the friction between wheels and tracks—which is a major source of energy loss in traditional high-speed rail. This makes them exceptionally efficient at low to moderate speeds. However, as the train accelerates toward 500 km/h (310 mph) or more, aerodynamic drag becomes the dominant consumer of energy, regardless of the levitation system. At these extreme speeds, a maglev train requires massive amounts of electricity to overcome air resistance. In a "net" comparison, a maglev train operating at the same speed as a traditional high-speed train (like the Shinkansen) is often more energy-efficient because of the lack of mechanical friction and the use of "regenerative braking," where the magnets recapture energy as the train slows down. The true "green" potential of maglev is currently being explored in "vacuum tube" or Hyperloop concepts, where removing the air (and thus the drag) would allow the magnetic levitation to operate with almost negligible energy consumption over long distances.