Unfortunately, some of the TransDEM textures appear as built-in content in TS12. Apparently this happened at Auran/N3V by accident, as one of the routes delivered with TS12 didn't have its TransDEM textures removed when added to the Trainz asset data base.
This partly built-in and partly not of TransDEM textures causes confusion for route builders using TransDEM. I asked N3V to remove those textures from the built-in content but that hasn't happened so far.