Electric railway that runs on a single rail either above or under the railway cars. The first systems were introduced in the early 20th century; the earliest probably opened in 1901 in Wuppertal, Germany. Short-run monorails have since been built in such cities as Tokyo and Seattle. Because of higher costs and slower speeds than conventional rail systems, the monorail has not gained wide support. High-speed monorail vehicles that use magnetic levitation have been undergoing research for many years.