Are you asking aboute prototype or the model?
Here are some easy rules of thumb.
Drag freight (level ground): 1/2 hp / trailing ton
Time freight (level ground): 1 hp / trailing ton
High speed freight : 2.5 hp / trailing ton
High speed reight (grades): 4 hp / trailing ton
The hp / trailing ton for freight should equal the grade (a 2% grade should have at least 2 hp / trailing ton).
So a GP20 should pull 4000 tons on straight and level, 2000 tons if you want it to hit 50 mph, 800 tons if you want it to hit 70 mph and 500 tons if you want it to hit higher speed in the mountains. If you have a 2% grade, you don't want more than 1000 tons on the train.
Real tonnage ratings will vary, but those are good for quick and dirty estimates.
Dave H. Painted side goes up. My website : wnbranch.com
Got yet another one for the forums: What is the "normal" tonnage for a GP20? In other words, on straight and level track, what would be the maximum that a 20 would pull? I have some non-powered 20s that I am debating installing power in. (And yes I have either already have or managed to locate the needed parts to power the drives that I have for the dummy units. In most of those dummies, the trucks and wheels are already the right type for power.) I am trying to decide if what I have planned prototypically would require two units or if the units would run by themselves.