Drew,
Nice flats !!, and the weathering is great !
It reminds me of the daunting task I have before me,,, that of weathering, or more correctly, re-weathering most of my rolling stock.
I do try not to be a rivet counter, but I do count rivets. There are details that I will strive to include on models that I build, and the attention to detail gets more accute as time goes on. So, please understand that I am NOT criticizing. My intent here is purely educational.
Wheels: To rust, or not to rust, that is the question. Really, when do wheels get rusty? This depends greatly on the type of bearings used in the trucks.
Roller bearing trucks do not get regular applications of lubricant. the bearings are sealed, and usually don't leak, so there isn't any lubricant on the wheel faces, and they will show rust through the dirt.
Friction bearing trucks have the doors lifted, and oil added constantly! The oil leaks easily from the journal boxes, and centrifugal force spreads the oil quite evenly across the face of the wheel. This oil, and all the dust and dirt combine to form a dark sludge on the wheel face.
The wheels of friction bearing trucks will be rusty; if they are newly mounted in the truck frame, and haven't had much "run time", or, if the car has been sitting idle for an extended period of time (months). Otherwise they will be dark.
The wheels of roller bearing trucks will be a bright rust color if they are new, and a dirty rust if they've been in service.
I wish I had learned this about a thousand wheel sets ago, I wouldn't have the major project to do now, that being, to remove the rust from the steam era wheels, and add the rust to the modern wheels.
Considering the age of your flats, and the hard use they get, I'm sure the wheel sets have been changed several times, and it's been well over a month since they were last done. My opening comments stand, nice flats! and the weathering is very nicely done.
Pete