I agree with Don, fuel consumption and wind resistance were non issues during the heyday of F units.
Elephant style would also assume a means of turning the locomotive, ie a loop, wye or turntable. Back to back enabled the consist to return back to is home without turning the units. Esp important on branch lines with no means to turn or where the turntable was too short for an AA set. Some railroads acquired F units draw barred together to get around union crew requirements. Draw barred together meant they counted as only one locomotive, but prevented elephant style. In later years, draw bars were removed to allow units to be used individually, allowing for better power balance. That is when elephant style became more common.
Doug Harding
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