Prototype maglev, (
magnetic levitation)
trains have been in development for over 20 years now, with cruising speeds up to 400 miles per hour. The advantages of maglev include high speed, very low friction, high efficiency, and low air and noise pollution. The disadvantages of current systems are high maintenance costs, complexity, and operational stability.
There are three magnetic levitation technologies being used in current prototypes. Electrodynamic, in which superconducting magnets in the guideway levitate the car; Electromagnetic, in which superconducting magnets in car lift the car; and Inductive levitation using permanent room-temperature magnets in the car and a special track.
The disadvantage of both the Electrodynamic and Electromagnetic systems is that they require superconducting magnets, which use superconducting coils to generate their magnetic field. These coils must be electrically powered and require expensive, cryogenic cooling systems, and complicated feedback circuits to prevent instabilities in high-speed use.
The inductive system uses permanent room-temperature magnets mounted on the car and a track which has close-packed coils of insulated copper wire embedded in it. The permanent magnets are arranged in a Halbach array, (named after Klaus Halbach a retired Lawrence Berkeley National Laboratory physicist). Halbach arrays concentrate the magnetic field on one side, while canceling it on the other side. When placed below the bottom of a rail car, the arrays generate a magnetic field that induces currents in the track coils below the moving car, lifting the car and centering it over the track. The inductive system requires no power to the magnets or the copper coils, but it does have a high initial drag, which increases as the train accelerates from a stop until it’s transition speed of about 2 miles per hour is reached, after which the drag decreases as the train speed increases.
(Note: the main drag operating in all magnetic levitation systems is aerodynamic drag.)
NASA is spending $150 million to develop a prototype Inductive levitation system in the hope it can reduce the cost of rocket launches.
Given the advantages of any type of magnetic levitation I am somewhat surprised that commercial development is not further along.