The EMD SD40-2 and the SD40-3 upgrade makes up the majority of the Rapid City Pierre and Eastern Railroad, so the best place to start then is to figure out just who makes a model of SD40-2.
For those who don't know, here is what an SD40-2 looks like:
These engines are a traditional EMD hood unit with a standard cab. Like all SD type models, the SD40-2 has 6 axles compared to the standard 4 found in GP units, which look similar but are shorter. SD40's have the distinguishing feature of a large deck on the front and back of the unit giving it a frame that is nearly 68 foot, 10 inches long. The SD40-2 has a 3,000 horsepower diesel engine that made the model the most popular road switcher in EMD's catalog with nearly 4,000 being produced in the production run, which lasted from 1972 to 1989, quite an impressive run.
The Rapid City Pierre and Eastern has 60 of these units with the only other type of engine being a trio EMD GP38-3's.
So then, who makes a model of the SD40? Well here's what I've uncovered:
Athearn, Bachmann, Broadway Limited Imports, Intermountain, Bowser, Kato
Midwestern Model Works, Lionel, MTH
So it seems that HO scale offers the most choice, naturally, but an SD40-2 can be acquired in all the popular scales.
The next step will then be to decide what scale to model in! That's the hard part!