As you know, it's a fictional locomotive, but the interesting thing (at least to me) is that it incorporates features similar to a variety of other locomotives. However those features are from vastly different locomotives from different manufacturers. Perhaps that was their intention, to have something that is not a "copy" of something, but it does make finding a hobby equivalent a little difficult. What scale are you looking for? To run or just display? And how close do you want it? i.e. Out-of-the-box similar? Or are you planning on modifying the body or kitbashing it to look as close as possible?
The first thing that sticks out to me are the noticeable "porches," which are the platforms at the front and rear. The locomotive most like that is the SD40, which ended up that way because it used the same frame as the larger-engine SD45, but under a shorter lengthwise hood. The result was extra space at each end, which was left as a platform:
However, the SD40 is a six-axle locomotive, versus only four in your image. I can't think of any four-axle locomotives with a noticeable overhang like that. Also, the trucks on the FarmSim locomotive are a little strange; they look like modern hi-adhesion trucks, but with only two axles. AFAIK, hi-ad trucks are only made with 3 axles each:
Next, the cab has a mix of window designs. It has two half-width front windows, which is practically non-existent on that style of cab, because most have an entry door on one side of the front cab. A long windshield like that wouldn't permit a door next to the nose. Plus, the front windshield is angled. That makes me think of an Alco, such as a C420:
The side cab windows are a little unusual in that they are 4-panel square windows. That makes me think of the cabs specified by some Canadian railroads, such as on this rebuilt GP20C-ECO:
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I think that has its origins in the early Canadian designs of the "North American safety cab," e.g. in this C44-9WL:
Note the bell placement and trucks...similar to the FamSim train.
Finally, you've got flared radiators in the middle (similar to a GP40 or 50 with dynamic brakes), and five EMD-style top fan assemblies (SD40, SD45...I don't think any 4-axle GP's had 5 top fans).
So I guess it comes down to what elements you want to match closest. If you want the front porch, try an SD40. If you need four axles, look at the GP40/50/60. If you want the angled front windows, check out some Alcos, C420s, C628s.
But y'innow after all that...I just had a thought. Similar color, front and back overhang, four top fans, mid radiator flare. How about an Elgin, Joliet, and Eastern SD38-2?
This was a six-axle, somewhat limited production version of the GP38. Because they stretched the frame to fit the extra axles, it has porches like the SD40. Only thing you'd have to do is paint over the logos and lower frame, and re-number it. Kato, Athearn, and a few other manufacturers made this in HO scale. I think Intermountain has one listed for future production (but the ways things are moving lately, could be years before it's available for purchase).
Good luck to you, thanks for giving me a way to avoid being productive around home for an hour this afternoon.
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