Right now, if it's really grotty, you want an abrasive block like a
Brite-Boy to clean the rail head. A wire brush in a moto-tool can be used to clean the web of the rail if it's really ugly, but it's the rail head you need to worry about for running trains. You don't want to use anything too abrasive, like sandpaper, because you'll score the rail, and just make more places for crud to collect and the rail to oxidize.
Once you've got it cleaned, the best thing you can do is use a conductivity enchancer like
Rail Zip to treat the rail and minimize the rate of oxidation. People have apparently had good results with clipper oil, but I've never tried it myself.
I hope you didn't pay too much for it; brass track is, well, a pain in the brass. It oxidizes fairly quickly, the oxide doesn't conduct electricity and so it needs constant cleaning.
Nickle-silver is much better for the simple reason that the oxide is conductive - so even though the rail may be oxidized, your trains will still run on it. It also requires less-frequent cleaning.