Just do as xcrossi says. Open assets for edit in explorer, click once on your Trainz install folder and search for env_metal1.tga. It will find many. R-click on any of them, select "open location", copy that and the env_metal1.texture.txt file to a known location (I have a folder set up called Trainz Bits on my C:\ root just for this purpose). Then, copy both of these files into the each asset in turn. To do this, open each asset in turn, copy the files over (into the right folder if called for, other wise into the parent folder of the asset). Close the asset, move on to next. When you're done, select them all, R-click, Edit, Commit. That should sort you out.
Hope this helps. Regards,
Ken