To do it on TC3, you need to delete the assets.tdx and Cache/assets.bku files from the Trainz install folder whilst Trainz is not running, then re-run Trainz itself to rebuild the database from the info in Local and the built-in folder.
This is strange. I can see the .tdx file ok, which I assume is the local content, but nowhere in the TC3 folder is there any .bku files, so I'm a bit baffled. maybe I will need to do a re-install of TC3. I have all my downloaded content saved on a flash-drive, so that not a big problem, but its a right old job doing all the TC3 patches again :-((
The .bku file may not be present. Just delete the assets.tdx file, then re-launch Trainz itself and wait for it to load. The actual local content is stored in the Local folder.