In the US a railroad can go from having not enough motive power to having them in storage because of econmic conditions.
Due to wheel to rail loading, to much horsepower to weight can cause wheel slip for a given amount of weight, also rail size, ie 90lb verses 130lb etc limits wheel to weight ratio. Generally the finding is 4500 to 6000 hp per unit mainline depending if it is a drag or fast freight, a lot of other factors also enter into this, ie gradient degree of curvature etc.
Train length depends on the RR Don't know what it's like now but SP would run long heavy drag freights, and Santa Fe 'BNSF' would run smaller faster freights.
UP through Tucson AZ. Usually runs 2 Headend units and 1 or 2 rear units depending. Units will range from 4500 to 6000 on average, most units being in the 4500 catagory.
Having more units at say 4500hp than one unit at lets say 9000hp gives you more options as to power requirments. It also gives you the benefit that if one unit in a power consist does not work it can be replaced.