Tume, I believe the part that you marked in the last picture is the fuel filter. I may be wrong but I'm pretty sure that's what it is. Great looking SD-40-2!
Have a good day,
Bernie
On my CNW C44AC that I detailed in HO scale, it was actually an air filter. Not quite sure how an air filter got placed under the frame just in front of the fuel tank, but thats what the brass part was called when I bought it. Same location and design, I would assume its the same part. It may be an intake for the compressor perhaps?