Aerial Mono-Flyer of the Future (1918)
The August, 1918 cover of Hugo Gernsback's Electrical Experimenter magazine featured the "aerial mono-flyer of the future."
This monorail seems like only a modest improvement in safety over the 1930's sightseeing death-trap known as the sky toboggan. But the mono-flyer is assuredly a less safe concept than the monorail of William H. Boyes, built around 1911.
This image was found in the book Out of Time: Designs for the Twentieth-Century Future. According to the book, Gernsback introduced Electrical Experimenter in 1915 and changed the name of the magazine to Science and Invention in 1920.