It works soaking them in motor oil too. Gramps compared cleaning rusty train metal to cleaning his Garrand...Agitation is the key. Oil is protection and solvent both. You could always see the oil sheen on Gramp's metal 
Let the excess drip off, set on a towel overnight, wipe lightly, .. then you're golden.
Sometimes it takes time & repeating (weeks at first,then months/years) of a little oil and agitation to remove deep rust 100%, and some still black metal is partially oxidized and will never shine again, may or may not rust easily; but mostly don't rust if the metal isn't stripped of oil for long or let to varnish while dirty)rusty.
