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Quick answer
There is no fixed number. A freight train may use one locomotive for a light local move or several locomotives for a long, heavy mainline train. Railroads match locomotive power and tractive effort to train tonnage, grades, curves, desired speed, weather and operating requirements.
Why the number changes
A locomotive has to do more than get the train moving on level track. It must control the train on grades, maintain an acceptable speed, handle curves and changing conditions, and provide enough braking capability. A heavy unit train climbing a mountain route may need much more locomotive capacity than a lighter train on flatter territory.
Horsepower is only part of the answer
Horsepower helps determine how quickly a locomotive can do work at speed, while tractive effort describes pulling force at the rail. Adhesion, axle count, gearing and locomotive weight also matter. That is why railroads consider train performance rather than simply dividing train weight by a horsepower number.
Why locomotives may be spread through the train
Distributed power places locomotives farther back in the consist instead of concentrating every unit at the front. This can reduce in-train forces and improve handling and braking on long or demanding trains.
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