Here's a good look at the roof of a New York Central Consolodation-Line B unit.
The fan housing seems rather flush with the roof here but I've seen some C-P engines that seem to have a slightly taller dynamic brake fan housing.
NYC, Dayton, Ohio, 1956 by Center for Railroad Photography & Art, on Flickr
Click the photo to go to the Flickr site and you can get a closer look at the grating and fan. Not all roads ordered the dynamic brake option. I believe Long Island was one of them.
The Proto 1000 model has rendered the fan and grille fairly well:
NYC C-Line F-M DB by Edmund, on Flickr
NYC C-Line F-M by Edmund, on Flickr
Good Luck, Ed
Not many top-down photos of these old babies to be found, but here are a couple that come close:
https://www.railpictures.net/photo/353971/
https://www.railpictures.net/photo/282385/
http://www.rrpicturearchives.net/showPicture.aspx?id=5401087
Yes, that's supposed to be a fan; it and the grid mean the locomotive is equipped with a dynamic brake (explanation below if you're not familar). The AHM shell is pretty crude on detail; it's represented a bit better on the old Life Like Proto 1000 models. Here's an eBay listing that shows it:
https://www.ebay.com/itm/154243369261
On a C-Line locomotive without a dynamic brake, both elements will be missing, as in this loco:
https://nycshs.omeka.net/items/show/117506
In case you're unfamiliar, the dynamic brake turns the traction motors into generators, which feed power into resistors; the electrical resistance slows the motors and (by extension) the train. The power generated is wasted as heat, hence the fan and grids. It was an optional feature in most diesels, and tended to be ordered by railroads operating in mountanous territory and not so much by the flatlanders.
Modern hybrid and electric cars do something similar -- regenerative braking, with the energy fed back into the battery. I belive electric trains use regenearative braking, feeding power back to the caternary/third rail.
"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."
On the PRR's CFA and CFB 16-44's there are vents in roof behind the cab, flanking a round structure, which I presume is a fan housing. But on models it is just rendered as a round, flat topped protuberance with no grating or screen So what is this thing? Is it just a very poorly rendered fan housing or what? Does anyone have a picture looking down at the top of the prototype?