if its invisible did you export the textures wwhen you exported the gmax model? If you did check that the texture files are not compressed trainz doesnt like that. Also check the size of the textures they need to be
32 x 32
64 x 64
128 x 128
256 x 256
512 x 512
1024 x 1024
or you can mix these as long as one side is not greater the 2 time the size of the other. i.e. 128 x 64 or 128 x 512 etc.
The other reason it could be invisible is the config file there may be a typing error or a bracket missing.
If you still cant fix it email me your file and I'll sort it out for you
UKBL@talktalk.net
What version of trainz do you have? if its TRS 06 the just right click on your item in CMP the click on save as a cdp
Hope this helps
Andy