Although railroads revolutionized society, their speed and efficiency were hampered by the nature of their motion. Friction generated between the wheel and the rail limited this method of transportation. This problem was cured with the advent of the Mag-Lev or the Magnetic Levitation Train. Magnetic levitation used repulsive magnetic forces to lift trains one foot above the rail, allowing faster speeds and reduced emissions. Japanese advances in superconductivity introduced this prototype in the mid-1980s, later replacing millions of miles of railroad in the second half of the 21st century.