Depends what you are looking for... There is a lot of great things out there but generally if your looking for high quality sounds, details etc, you might want to look into payware. Some stuff like Maple Leaf Tracks, 3dtrains, and facingpoint are very good but you have to pay for most of the really good stuff . If your looking more into the freeware part then NALW has good stuff and some other private builders. Is there anything certain that you are looking for?
Edit: See my Train-sim links above.