An image of a 'double truck'. This truck has everything that a single truck has (four equal size tires and a motor on each axis), but works in conjunction with a second truck to move the trolley. The trucks hold the trolley base and allow it to roll on the rails.
International Correspondence Schools A Textbook on Electric Lighting and Railways (Scranton, PA: International Textbook Company, 1901)
Courtesy the private collection of Roy Winkelman