Maybe I'm a bit biased to older, historic RR subjects but I feel the fist generation EMD Geeps (GP7 and GP9) and and SDs (SD7 and SD9) are a little underrepresented in LEGO MOCs given their historic nature. Fortunately a two builders out there have decided to model these history making locomotives and they've done fantastic job.
Aberdeen & Rockfish RR GP7
On the Geep side we have an Aberdeen & Rockfish GP7 from Bricknerd.
The Aberdeen & Rockfish Railroad colors on this MOC work wonderfully together. I also love the brick built truck side frames that Bricknerd has come up with to to cover the standard LEGO ones. It really helps add a nice touch to the model. And I love that he's built a Geep with out the Dynamic Brake option. These were a less common sight especially since many early Geeps originally built without them were upgraded later in life to dynamic brakes.
Be sure to check out Bricknerd's Flickr Gallery. He has some other great Train MOCs as well.
Union Pacific SD7
And championing the SDs we have Swoofty's magnificent Union Pacific SD7
Swoofty builds some of the most detailed 6 wide diesels and this SD7 continues that fine tradition. The vents, grills and dynamic brake involve some very clever engineering and I like the use of Technic pistons for the air tanks. And the Union Pacific always had some nice color schemes for their diesels.