:These are my suppositions, not checkable fact, so we should find some references before jumping off on it.  But, I think, the FP series were intended as dual-service units, as fully capable of freight service as the corresponding F unit.  They were just a lengthening for extra steam generator water / fuel capacity.  Additionally, it is well known that railroads who had to climb mountains with heavy loads preferred F units to E units for the purpose, because of more traction motors (less possibility of burnout) and all the weight being on driven wheels.  They were also used on short-haul services, where they would have been more practicable than an E unit. 

 :We should see if we can find any references that state this.