Trainz Railroad Simulator is the general product--there is TRS 2004 and TRS 2006 (What I use). There is also a newer version of Trainz called Trainz Classics, but from what everyone has said about it, you may want to keep away from it for a while...I don't have it, and am totally satisfied with 2006. I've seen Trainz Railroad Simulator 2006 at Wal-Mart (where I got my copy).
Tim