I have previously discussed how I ballast C track. Here is technique for weathering that I have found quite useful.
After ballasting the track I spray the whole thing with acrylic spray from NOCH (NOCH #61173). I use the colour brown to give the appearance of rust and dirt on the track. This also helps colour the rails in a nice rusty brown.
Tip: The NOCH acrylic sprays comes in smaller 200 ml cans. The larger cans from the DIY store contains 350 ml or more. The NOCH cans take up less space, so you can store them more easily together with your hobby items. The price pr. liter is around the same as the larger cans. So you do not pay more for less. The paint is made by German Dupli-Color.
I then give the whole thing a wash with Pewter Patina Finish from Woodland Scenics (Woodland Scenics #M126). This wash flows extremely well, even better than the Vallejo washes that I have previously used. I highly recommend this product.
When done, the track looks awesome. I still dry brush it with some ivory colour from NOCH (NOCH #61191) to give it some texture. You can also spray this on with the acrylic spray of the same colour, but that takes practice not to mess up. I have found that dry brushing is the safest way to go.
If you are weathering C track, give this technique a try. I think you will be happy with the result!