Over the past four years, I have contributed 1000 items of custom content to the Trainz Download Station. These include ground textures, buildings, rolling stock, and various scenic accessories.
Early models were made for TRS2004 and I understand that most can be used in later versions, although I have not checked all of them personally.
From September 2010, support for TRS2004 was withdrawn by Auran but this content continues to be available from the Download Station.
My recent and current contributions to the Download Station are for Build 3.1.