There could be many versions, but one that comes to mind is a corrupt database. Since we don't know what version of Trainz you're using, we can't help you because the procedure for TRS2006 and Classics is different than TS2009 and above.
I had the same problem a few years back trying to run 06 on a laptop with built in graphics that did not support OpenGL which Content Manager requires. I traded the laptop for a desk top with more memory, installed a decent graphics card (nVidea) and the problem disappeared.
There could be many versions, but one that comes to mind is a corrupt database. Since we don't know what version of Trainz you're using, we can't help you because the procedure for TRS2006 and Classics is different than TS2009 and above.