I have a tree generating script for gmax on my website ( use my signature link to get there, then go to the Maxscripts page) that might work for you. You can generate your tree in gmax, assign it a placeholder .bmp texture and export. When you import it in to Trainz, just follow the location and texture size conventions of that editor. The generator script works nicely, but be careful about how many ploys you end up with in your tree... lower is better!

Two things to check :
1. Make sure your texture is the correct size. It needs to be divisble by 8. Ex: 128 x 128, 256 x 256.
2. If the texture is a TGA file, make sure that it is uncompressed.

These two things are the biggest reasons why you get a white object in Trainz.