A knife sharpener uses a gasoline engine to power his mobile grinding rig, which is on a trailer attached to his bicycle. When he’s not sharpening knives and scissors, the small businessman attaches the engine to the trailer wheels, so motorized trailer pushes the bicycle.
In this photo from the November 1937 issue of Popular Science magazine, the man is shown filling up at a service station. From Modern Mechanix blog.
They make motorized push trailers so much smaller and cuter these days!