I'm posting this here, because the example happens to be HO; but it could apply to any scale.
The recently acquired Alco Models RSD4/5 showed typical tarnish from being 40-plus years old. I did a couple of old candle-stick holders using an old stand-by; "Bon Ami", so I thought I might try it on this loco.
I haven't done the short hood yet; here is how it's looking so far:
Just thought I'd pass this on to those (like me) who have to rely on something other than an ultrasonic cleaner...
Mark in Oregon