When the 645 engine entered production in 1965, a large series of new locomotive models was introduced. The turbocharged version was used in EMD's "40 Series" (GP40, SD40 and SD45) in , sixteen-cylinder form and in , twenty-cylinder form. EMD also introduced the Roots-blown "38 Series" (GP38, SD38) and turbocharged, twelve-cylinder "39 Series" (GP39, SD39). All of these locomotive models extensively share common components and subsystems, thereby significantly reducing cost and increasing interchangeability. The GP38-2 and SD40-2 became the most popular models of the series and among the most popular locomotive models ever built.