turbo-compressor (turbocharger) with intercooling. The introduction of the EMD-type turbocharger was successful and all subsequent SD series were offered with this turbocharger, although not all models within a series were offered with turbocharging (e.g., the 38 sub-models within the 40 Series were Roots-blown). Options Optional equipment that could be specified by ordering railroads included multiple unit controls, a steam generator, dynamic brakes, winterization equipment, an air signal line, and hump control. Despite the options the vast majority were similar in configuration. The standard fuel tank offered a capacity, while tanks were optional. In practice, all SD24s ordered had the tank except for the last built, Kennecott Copper's single locomotive. In order to provide room for a larger fuel tank, the air reservoirs were relocated on the roof just behind the locomotive's cab. The tanks were known as "torpedo tubes" due to their long, thin design. Winterization included a winterization hatch over one of the radiator fans, to direct warm air back into the engine compartment. Either a low or high short hood could be ordered; in either case, the short hood was front by default. The Burlington, Southern and first EMD demonstrator had high short hoods; all others were low short hood. Low short hoods from the factory had a pronounced downward slope of the top towards the nose, as did those of the contemporary GP20. Many of the high short hood units had their noses chopped later on, often as a consequence of rebuilding; these as a rule have flat-topped short hoods. Original purchasers Rebuilds As the SD24 locomotives aged, they began to develop reliability problems, especially electrical ones. In addition, the extra maintenance requirements of the turbocharged engine were acceptable when they were some of the most powerful locomotives available, but in secondary service, they were an expensive way to get . Thus, a large number of SD24s were rebuilt by various owners to extend their lives. Illinois Central Gulf SD20 From August 1979 through December 1982, the Illinois Central Gulf Railroad rebuilt various C-C locomotives (the EMD SD24, the EMD SD24B, the EMD SD7, and the EMD SD35) to 2000 horsepower roadswitchers powered by EMD 645E prime movers, resulting in the EMD SD20. All units had the electricals upgraded to Dash 2 technology, and the turbocharger was removed on the SD24s and SD35s. 35 of the 42 SD20s built were rebuilt from either the SD24 or the