Many years ago I recall reading an article about a guy who rigged his bike to ride on abandoned train tracks. There was an outrigger with a wheel on the end that rode on one rail with the bike riding the other. The front wheel had an attachment with rollers that kept the wheel on the rail.
One of the things that intrigued me about the idea was the authors observation that train tracks never have more that a three percent grade; that would be like riding on a sheet of glass, albeit a very think piece.
Anybody read, heard or do such a thing?