Monorail

A monorail is a variety of mass transit where vehicles travel along a single rail. There are two main varieties: where the vehicles rest on the rail, and where the vehicles are suspended from the rail. The largest monorail network yet constructed, that in Wuppertal in western Germany, is of the suspended type.

On the Wuppertal monorail system: http://schwebebahn.com/en/index.asp