Electro-Motive Diesel Inc. really hit a home run with the SD40-2 model. At 3000 hp, the SD40-2 had the power, and with 6 axles, the engine performs flawlessly on bumpy track. Together, the platform boasted an impressive 80,000 pounds of tractive effort, 92,000 starting. Of course, that's not huge by today's standards, but turn the clock back to the 1970's, and it's easy to see why the SD40-2 is loved by railroads big and small.
An SD40-2 is instantly recognizable due to the large decks fore and aft on the locomotive due to the 69 foot frame, most other SD series engines were based on a 65 foot frame or shorter. There is also the standard low nose cab on most models, though a few had a high nose (Norfolk and Western). The SD40-2 featured HT-C engine trucks with improved motors and a better ride than the older flexi-coil design seen on the first generation models.
Please see this excellent webpage on the SD40 series engines: http://www.american-rails.com/sd40-2.html
SD40-2 Road numbers: 6401-23, 6427-33, 6436-37, 6439-46, 6448-59
SD40-3 Road numbers: 3420-33, 3480, 6051-52, 6055
Total units: 63 (now 60)
***Road numbers: 6406, 6429, 6444 sold to Arizona and Eastern Railroad***
The GP38 series of engines were quite successful on their own. Lacking the six axles of the SD series, the GP's were somewhat more nimble in switching operations. However, the GP38 was only rated at 2,000 hp, meaning that it wasn't quite as strong as it's other GP and SD cousins, though the 38 was much stronger than the 7 or the 9 model.
Like other EMD GP units, the GP38-3 is instantly recognized by the 4 axles and large ventilation duct on the right side of the unit, just behind the cab. Road-specific details varied greatly, meaning that no two sets of GP units were quite the same, railroad to railroad. While heavily upgraded, the GP38-3 was hardly any different from the GP38-2 for exterior features.
Please see this excellent webpage on the GP38 series engines: http://www.american-rails.com/gp38.html
The Rapid City Pierre and Eastern has 5 GP38-3 units, 2085-89.
Considering the modest mainline speeds, and the fact that the entire roster of the RCPE happens to be road switchers, all these engines can perform well anywhere on the system. However, the GP38-3's are more suited to working near the large industry operations, due to their limited range on the mainline. Huron, Pierre, and Rapid City would be the home-base for the GP38-3 units.
As for the SD40-2 and SD40-3 units, they can be found anywhere on the system at anytime, even abroad on different railroads like the BNSF or CP Rail near the interchange with the RCPE system.