I used to shoot the whole track with a color called sweet chocolate that came in a rattle can from K-mart. (In truth, any similar flat brown will do.) It does a pretty good job of simulating creosoted ties and rusty rail, but has to be done before ballasting, so is out of the question for you. Since you're going to be brushing, mix up something of a dull, dark rust color that looks like it's been there a long time. Colors like the rust that Floquil made are too bright, too orange and simulate rust that's too fresh. BTW, the Noch grass mat looks very good.