A vactrain (or vacuum tube train) is a proposed design for very-high-speed rail transportation. It is a maglev (magnetic levitation) line using evacuated (air-less) or partly evacuated tubes or tunnels. The lack of air resistance could permit vactrains to travel at very high speeds—up to 4,000–5,000 mph (6,400–8,000 km/h), which is 5–6 times the speed of sound in Earth's atmosphere at sea level—using relatively little power. Vactrains might use gravity to assist their acceleration. If these trains achieve the predicted speeds, the trip between Beijing and New York would take less than 2 hours, surpassing aircraft as the world's fastest mode of public transportation.