Monorail

Noun

 * 1) a railroad system where the trains run on one rail
 * 2) a train running on a single rail

Thesaurus
Amtrak, L, baggage train, branch, cable railroad, cable railway, choo-choo, cog railroad, cog railway, el, electric, electric railway, electric train, elevated, elevated railway, embankment, express, express train, feeder, feeder line, flier, freight, freight train, freighter, funicular, goods train, gravity-operated railway, horse railway, interurban, junction, light railroad, lightning express, limited, line, local, main line, metro, milk train, parliamentary, parliamentary train, passenger train, rack railway, rack-and-pinion railroad, rack-and-pinion railway, rail, rail line, railroad, railroad train, railway, rattler, roadbed, roadway, rolling stock, shuttle, shuttle train, sidetrack, siding, special, streamliner, street railway, streetcar line, subway, switchback, terminal, terminus, track, train, tram, tramline, trestle, trolley line, trunk, trunk line, tube, turnout, underground, way train

Etymology
From the monorail:.