An Elevated railway is a railway built on supports over other rights of way, generally city streets. They are also called Elevateds or the el. This was a popular way to build mass transit systems in cities around the turn of the twentieth century, but it has fell out of favor and many elevated lines were later demolished, being replaced by subways or buses. Some new elevated lines have been built in the late twentieth century, including monorails and the AirTrain In the United States, only the Chicago 'L' is known by that name ('L' is short for elevated).