Very powerful electromagnets.

Magnetic levitation (maglev) is a relatively new transportation technology in which noncontacting vehicles travel safely at speeds of 250 to 300 miles-per-hour or higher while suspended, guided, and propelled above a guideway by magnetic fields. The guideway is the physical structure along which maglev vehicles are levitated. Various guideway configurations, e.g., T-shaped, U-shaped, Y-shaped, and box-beam, made of steel, concrete, or aluminum, have been proposed. http://inventors.about.com/library/inventors/blrailroad3.htm http://staff.wcasd.net/teachers/jmaxwell/magnetic_levitation.htm http://encarta.msn.com/encyclopedia_761579465/Magnetic_Levitation_Train.html Maglev is a system in which the vehicle runs levitated from the guideway (corresponding to the rail tracks of conventional railways) by using electromagnetic forces between superconducting magnets on board the vehicle and coils on the ground. Principle of magnetic levitation The "8" figured levitation coils are installed on the sidewalls of the guideway. When the on-board superconducting magnets pass at a high speed about several centimeters below the center of these coils, an electric current is induced within the coils, which then act as electromagnets temporarily. As a result, there are forces which push the superconducting magnet upwards and ones which pull them upwards simultaneously, thereby levitating the Maglev vehicle. http://www.rtri.or.jp/rd/maglev/html/english/maglev_frame_E.html Superconductive magnetic levitation transportation system http://www.delphion.com/details?pn10=US06418857

Too much electricty.Not efficient enought yet.

Like many innovations, Money, Time and Opportunity. Lots of each.