I went with Perma Blue to chemically blacken my tracks. I've also read that Carr's Metal Black works well, though i'm not sure which one you would use for Impact tracks, as i'm not sure what alloy the tracks are made of.
It takes some experimenting, but the effect is fantastic once they're finished.

I'm using Mig Pigments and their Thinners for Washes to weather them, and it should look great when finished.
Cheers,
Rik