I am late to this, but my preference is to use isopropanol (100%), I use it in manufacturing some of my products so I always have it on hand, I also have in stock xylene, toluene, Kero (as used in goo gone), Di-limonene, cold pressed orange oil, and a heap of other things that are not easily available to the general public and would blow some of your minds if you knew what they can do.
My favorite product for track maintenance is the isopropanol. There is no way any of the others listed above would go near my track, some are slow to evaporate and most of them can do some serious damage to the plastics and rubbers that could be exposed to them.
Isopropanal is residual free (unless it has dissolved some heavy contamination from a particular place and then it is redeposited right round the layout in a fine film, if you haven't noticed it) and quick to evaporate with out damage to any of the track features, but a word of warning if you paint the sides of your rail, it can cause paint too fall off over a period of time.
I also find the Tomix track vac car a very handy asset but only as a vac, I do not employ the abrasive discs. As most of you already know, Nickel Silver is an alloy of copper, nickel, and zinc so you will always be able to get evidence of a black residue from it when you rub it with a rag, it is not indicative of the track being dirty more so evidence of the oxide that forms on the surface of the alloy.
