I've have used Rail Zip, it should be used very sparingly. I normally just run a track cleaning wagon with a diesel now and again this is more effective. I've got two of the Roco ones which I modified, as with their normal springs I didn't have a loco which would move them. I then follow up with a centre line, with surgical fluid this works very well. Now and again I also use a soft rubber like that sold by the 00 association.