I think of a grinder, as in an angle grinder, for grinding down welds. Not at all the same thing as a polisher. Any polisher will do the rails, but you will need the right pads and abrasives to do a series of finer grits, perhaps going up to a 3000 grit polish like 3M's "Finesse" to get it mirror shiny. That might mean six different wheels, proper wool wheels perhaps, each charged with a different abrasive polish.
Stainless is always formed on carbon steel
tools, which embed iron flakes which always rust. So even the best of stainless needs to be passivated in an acid bath, which is not EPA-friendly, and is not cheap
, so it often is not done properly. There are passivating pastes on the market for DIY maintenance
. Using one of those will be most effective at keeping down the rust, if they are given a chance to "eat out" the embedded iron specks.
Or you can literally pull the rails and have them sent out for a proper acid bath, if you can find one.