That's simple. The TransDEM ground textures are provided with the TransDEM installation package, as a .cdp file. You must, however, explicitly import them with Trainz Content Manager.
The TransDEM Trainz manual has the instructions, on page 14. The manual will point out this necessary step in various other chapters, too. Don't worry, forgetting to import the textures is by far the most frequently made beginner mistake among TransDEM users.
During the preparation phase of TS12, Auran/N3V added a community-made route to the TS12 package. That route was based on TransDEM, and the route builder had not removed any TransDEM ground texture residue before loading it up. Since then, some of the textures have involuntarily become Trainz built-in content, to confuse new TransDEM users.