Was looking at Great Northern GP9s and found a paint scheme I liked. I created an okay looking drawing of how it may look. Actual model will look a lot better than this!
So the modifications that need to be done are:
- Change from four steps to five steps, as GN used five steps on their GP9s.
- Extra grab irons
- Roof-mounted bell - I do not know where I can find this, maybe could have someone 3d print one?
- 'Nail' antenna - Maybe I can fine one in brass from Des Plaines or P&D? Maybe 3d print as well?
- Leslie 3-Chime air horn - Is it an S3L or an S3KR? Cannot seem to find information on this.
- All-weather window(s) - Could use 3d printed one(s), if they are the same as the ones on GN GP9s.
- Kadee couplers or maybe Sergent couplers - Is there one with magnetic gladhands?
I'll take a crack at your list. Take my suggestions with a grain of salt, as I couldn't find much in the way of photos showing any of the rooftop details.
Steps - Des Plaines offers an etched detail sheet for gp9's, which includes radiator screens and shutters, as well as replacement steps. The part # is OSA-1057. I'm not sure how many steps are included in each set, so you may need two sets to get enough for a 5-step version.
Extra grabs - if you can't find a detail part from Des Plaines or P&D, you can use Tichy phosphor bronze wire to bend your own.
Roof-mounted bell - you may be able to use one of the roof-mounted bells from an Atlas switcher. I couldn't tell from photos exactly what the bracket and air piping arrangement was on the GN units, so I can't say for sure if the Atlas bell will match.
"Nail" antenna - again, without better photos it's hard to tell exactly what this looks like, but it would seem to resemble a firecracker antenna with a disc on top. If a part can't be found, this should be easy to fabricate from brass or styrene tube and rod, plus a brass disc for the cap.
Horn - I didn't dig too deep, but I did find a GNRHS article on detailing HO gp9's, and it listed the Details West AH-190 horn in the parts list. Per Details West, that would be a Leslie RSL-3l-r. An image can be found in the DW online catalog. It appears to be very similar to the PSC EMD 3 chime horn (PSC 5659).
All weather windows - Bill Davis of American Scale Models has the ex-Overland parts stock, and I know I have seen all weather windows on his site. I also know that somebody here on OGR recently described sourcing 3d printed parts from Shapeways. I don't remember the thread, but it had to do with Atlas EMD switchers. I have no idea if any would match the GN prototype.
Couplers - I don't believe that Sergent does O scale couplers. Kadees will work, or you can try Protocraft in brass or San Juan in plastic for nice renderings of Type E couplers.
Here's a link to a nice build thread starting with a Red Caboose model. There are some photos in there showing how the steps turn out. He also used the Protocraft couplers and there are photos of those as well.
For proto info, you might check with the GN historical society. Also check to see if Diesel Era ever did a study on the GN gp9. They seemed to cover each road with a photo study, and I'm pretty sure they did a series on the gp9.
Good luck, I hope this helps.