It's very paint-scheme and locomotive specific.
I do soldered-wire brass handrails on pretty much of everything, and I'll paint the shell up, do basic decaling where I can't reach, then put on the handrails and brush-paint the final rails and detail work.
One thing I would recommend is to drill all the holes, and do all the fitting for what you can (grabs), before that first paint. What I usually find is that excessive handling of the shell post-paint causes most of the problems so whatever steps you can do to simply handle it less is a good thing. So I'll do all the hole-drilling and detail fitting in the shell I can, test mount anything I can, remove what is in the way, etc. that doesn't interfere with decals, then paint, put the final decals and details on, touch up with a brush, then whatever final seal coat you choose.
My 'worst case' has always been the noses of my Santa Fe CF7's, with the nose herald under the vertical nose handrails, three side grabs, and also behind the sill handrails. Absolutely impossible to decal if the handrails are on first.
