One of the things taken into consideration would be the tonnage rating of the locomotives available. This is something that you can duplicate on your layout.
You need to make a "test train", which can be a random selection of the rolling stock that you normally use, or it can be a train of similar cars of similar weight. I use the latter - a number of Athearn 34' two-bay hoppers, all with "live' loads, giving a weight of 8 ounces per car, plus an Athearn caboose, which weighs four ounces. I assemble a train of any reasonable length, then attempt to run it up the steepest grade on a subdivision of the railroad. Chances are I'll have to remove some cars before the loco can take the train up the grade with little wheelslip. When the loco is able to do so, I count the number of hoppers, then multiply that number by the weight that I've arbitrarily assigned to these hoppers, which is 70 tons (20 tons tare, and 50 tons of cargo). The caboose is another 70 tons, for simplicity's sake.

If a loco can move 6 hoppers and the caboose up the grade, then I rate that loco, on this particular subdivision, at 490 tons. On another subdivision with a steeper grade or one with more curveature, the same loco might be capable of handling only 4 hoppers and the caboose. Therefore, its tonnage rating on this division would be only 350 tons. Not all of my trains are composed of loaded hoppers, and I've developed other formulae to account for lighter "general freight" trains, but the idea is similar.
So, using the examples above, if I need to move 10 hoppers and the caboose up that second, more severe grade, (a train weight of 770 tons), I'd need three locos like the one in the test example to do so. On the less-demanding grade, two locos would be sufficient.
To use this system of tonnage ratings, you need to actually test each of your locos, rather than rely on the fact that a prototype GP60 can pull more than a prototype GP9 - both of the models may pull the same amount. Once you've established your tonnage ratings, you'll be able to deploy your locomotive fleet more effectively.
Wayne