OK, now you are in business. When prototype wheelsets are to be remachined (I believe modern car wheels are single-use now), they are put in a wheel lathe between centers, and driven by a dog, or clamp. In our world, we can replicate that, sort of.
Check your bag of free, worn wheelsets and see if they are all the same brand. If not, then segregate them. Next, see if the journals (axle ends) are all the same diameter, probably 3mm. If the journals are badly scored or worn, put them aside, as they may not be worth rebuilding.. Now, make a brass bushing about 1/2" long with a drilled hole in it that is as good a running fit for the journals as you can make it. Put a drop of grease in that hole. This bushing gets chucked in the tailstock. Get the picture so far?
For the headstock end of the wheelset, you have two choices. First, hold that journal in the chuck jaws, or you can clamp the jaws on the flange. If you have collets, instead of using the chuck, that will probably give you more room to work. Bring the tailstock bushing up until it is lightly pressing against the wheel hub and lock the tailstock there. Now, the wheelset is supported on both ends. You will be working at the tailstock end of the wheelset.
Since you are an accomplished gunsmith, you probably can take it from here. My experience has shown that most wheels are a fairly snug fit on the axle/ insulating bushing, and light cuts can be taken without disturbing anything. To replicate the tread/fillet, I use a tool with a slight radius at the tip, and set the compound just under 3 degrees for the tread angle. While both wheels of a set should be remachined to the same diameter, variations between sets due to some being more worn than others will not mean anything, so just remove enough material to clean up the wear on a set by set basis. Any reshaping of the flange tips I usually do with a fine file.
Now, this technique, using a small lathe, does not make it possible to remachine the journals. This is probably only involved when fitting ball bearings to Accucraft brand trucks, and is another issue. My lathe chuck is large enough that the wheels fit thru the center hole and the jaws will close down between the wheel backs, clamping on the axle. So, I can machine the journals if required. Disclaimer--I don't think any of the above will work with ball-bearing-hub wheelsets.
Go for it!