From the TransDEM point of view, any georeferenced cartographic or aerial image can be used for both terrain ground textures and/or UTM tile textures. Technically, there is no difference. However, there are limits on ground texture resolution, just one pixel per 10 or 5m. That's why aerial images don't make much sense as ground textures if intended as a building aid. But the other way round, applying the map texture to UTM tiles, does not come with any such scale restrictions.
I think most of my examples in the TransDEM 2.5 announcement thread are UTM tile textures:
viewtopic.php?f=3&t=282This way, you could leave the terrain blank, when generating the Trainz from the DEM. In the second step you let TransDEM generate 3D UTM tiles and use the map textures for them. You will end up with the default Trainz grey and orange grid texture and have all options to manipulate it in Surveyor, with no other ground texture interfering.