I dont think the track has a shadow setting in your SCN file.
But to give that to all your track pieces would (i assume) be a big FPS hit.
If you do want to do it.
Add "ShadowCaster=(Static, Solid, 32, 32)" to all the lines in your SCN file relating to your track pieces (normally t0_s1.gmt, t2_s1.gmt etc., but NOT ta*.gmt as that is terrain)
I`ve never tried this, or seen it done, so it might not work or it might give rFactor some serious thinking to do lol
It would be interesting to see the results if you wouldn`t mind posting them?