The Rail Detail Products handrail set won't fit the Athearn GP60 shell because the Athearn GP60 isn't as long as the Rail Power Products GP60. That's because the Athearn model is merely a Phase II GP50 with a GP60 dynamic brake housing (the angled type). The prototypes are different lengths and that is reflected when you place the RPP GP60 next to the Athearn GP60. It's not much, but once you see it it's apparent.
The RDP handrails are simply copies of the Athearn design, though they are a bit more coarse. So, you wouldn't be improving anything by using them, even if you bent your own side rails to replace the too-long RDP side rails.
As 2slim mentioned, the Smokey Valley kit is available and it is worth it. You'll get some nice stanchions and pre-bent lengths of brass wire. Of course, I prefer to bend my own rails, but that's just the way I've been doing it for years.
There were some photos floating around the internet maybe a year ago when DW announced the GP60 detail sets and they looked fantastic. Of course, these were designed for the P2K GP60, so getting some parts to fit may take some work.
Here's a photo of a GP59 (actually, it's an Athearn GP60 model, but the prototype is a GP59) in NS OLS paint weathered to match the prototype:
All I did was paint up a factory painted Athearn model and add a few detail parts. As you can see, I need better handrails, too!
Good Luck! Can't wait to see the model!