Popular Science 1939 –
Walking on rollers that are geared to the rear wheels of a novel treadmill wagon, a child can propel the odd vehicle along the ground. Invented by Harry Kister, of Philadelphia, Pa., the cart is fitted with removable floor boards that cover the rollers, so that it may be used as a conventional express wagon for transporting small articles. When the wagon is turned upside down, the rollers are exposed, permitting the device to be used as a stationary exercising machine, which is strong enough for regular use by adults as well as by children.
Modern Mechanix Â» Treadmill Propels Child’s Wagon – Link.