In the past I used fine and extra fine steel wool for the fragile pieces. Once I purchased a large vibratory tumbler from HF it was a game changer. The mediums last a longtime and they're available at HF.
Pappy is correct, I use the 4 0 (0000) steal wool. is very fine and will leave no scratches. After is is clean I usually wipe down with lacquer thinner then clear the item.
Best results are done if the item has been removed from the train.
On larger couplers and items that are realty dirty I will uses a small brass brush or a Brass wire wheel mounted in a drill press, then use the wool.
I m sure that there are a lot more techniques and maybe i can also get some more ideas on this subject.
Ronh
Thanks very much