A monorail is a railway system in which the track consists of a single elevated rail, beam or track with the trains either supported or suspended. The term is also used to describe the beam of the system, or the vehicles traveling on such a beam or track. Many monorail systems run through crowded areas that would otherwise require the construction of expensive underground lines or have the disadvantages of surface lines. Aside from mass transit and people mover systems, a large number of smaller monorails have been built in amusement parks and at zoos.