I've seen pictures of an SD-40-3, and of an SD-40-2. Does anyone know what the major differences between the 2 engines are?? I can't really see any outward differences. Thank you in advance for the help.
The only major differences are all internal with upgraded electrical and controls systems. Many of the these so-called SD40-3's have no relationship to any kind of SD40 externally since they were rebuilt from SD-45's and look like SD-45's. The SD40-3 is an unofficial designation by the railroads that indicates upgraded standard SD40-2's electronics and control systems to newer computer cotrolled and monitored systems. For modelers, it means the chance to run SD-45 carbodies much later into the modern era than if the -3 uprgrades never happened.
As Jim says, SD40-3's are not a builder designation. They are all rebuilt from SD40, SD40-2, SD45, SD45-2, SD40T-2, SD45T-2 so they can look like any of those models externally. If you are doing a wrecked one, remember to put a 16 cyl engine in and not a 20 for those SD45's. You may also see SD40-2M, that is the same as an SD40-3, another rebuilder designation.
CN has a few SD40-3's. Here is a picture with a SD40-2 and a SD40-3 both in CN paint. http://rides.webshots.com/photo/2427205730061480584hCzFNj
As you can see they both look very similar. You can find quite a few of these in the Chicago/Northern IL area or Wisconsin and Minnesota. I don't know about the other areas CN runs in though. Oh and these aren't my pictures.
Seems everyone beat me to answering this. It follows the same though as UP calling an ES44AC-CTE, "C45AC-CTE". It's not a 4500hp unit, they just used 45 to tell them apart from the AC4400CW-CTE (UP: "C44AC-CTE).
I've heard rumors that the nearby (for me) Bessember & Lake Erie (owned by CN) is getting rid of their EMD Tunnel Motors and they are being replaced by CN SD40-3s.