A monorail is a railway with only one rail. The cars may hang beneath it, run on top of it (with a guide rail above or a gyroscope to keep them upright), or straddle it. The first was built in London (1824), the cars being horse-drawn. The first successful system was the Lartigue system: the cars straddled the rail and had horizontal wheels that ran on guide rails beneath. The SAFEGE system has cars hung from a box girder, split at the bottom, inside which the driving and guide wheels run. In the Alweg system the cars straddle a broad rail, on either side of which horizontal wheels run to provide stability. Monorails have been proposed as replacements for city subways.