 The GP9 became the most-produced EMD model ever, with 4,112 A units and 165 B units sold between 1954 and 1963.Owing to their ease of maintenance and versatility, most locomotives sold in North America since the introduction of the GP9 have been road-switcher, or "hood", units.Flush-sided locomotives based on a road-switcher chassis, or "cowl" units, would later be produced for passenger service.