I don't know about TS2009 because I rarely use it, but in TRS2006 when this happens as it did once to me because a file has become corrupted if you go to TRs2006 and delete the "assets.tdx" file, and go to Cache and delete the "assets.bku" file and then open Trainz, Trainz will rebuild those files, and that usually solves the problem. If you have a lot of assets, then this can take a while, so go have a cup of coffee or something while you wait. I am fairly sure there is a similar procedure for 09.