monorail, railway system that uses cars that run on a single rail. Typically the rail is run overhead and the cars are either suspended from it or run above it. Driving power is transmitted from the cars to the track by means of wheels that rotate horizontally, making contact with the rail between its upper and lower flanges. In the maglev monorails currently under development, powerful magnets hold the car just off its track and propel it at high speed. One of the principal advantages of a monorail is the relative simplicity of its trackage in comparison with that of a standard railway. Monorails originated and still function as materials-handling systems, similar to traveling cranes, for use in large factories. Short urban transportation monorails have been built in Houston, Seattle, and Germany. A maglev line linking Shanghai's financial district with its new airport began commercial operation in 2004.