Looks like it's for accessing the "Engine Oil Bath Air Filters".
I went to the Fallen Flags website, where they have copies of various "shop manuals" for locomotives. I found one for the SW1500. On page 1-1, it has a drawing with reference numbers for various items.
Ed
I'm coming back to this topic a bit later.
First, the only online photo I could find that showed anything concerning that area:
You can see a grey box in that general area, though on the wrong side. Maybe that's the filter in the drawing.
I do note that this special access hatch is extremely rare on these engines. That gets me thinking that perhaps it's related to putting in a different kind of air filter, one that needed more/different access. I wonder at a regular (non-oil bath) filter that might need replacement, on occasion.
A quick look at photos shows BN 380 also had a feature like this. There may be others. Maybe the new hatch is only "extremely rare" on non-BN switchers.
I looked at BN SW1500's, and didn't see any. Maybe they're more "crowded" in that area.
What is the purpose of the black hatch on top of the hood near the cab on this BN SW1000?
http://www.rrpicturearchives.net/showPicture.aspx?id=642694