So I was able to recover my old profile where the Trainz files were located but unfortunately the local folder is empty. I do see assets.tdx, asset-cache.tdx, content-store-task-list.td and keystore.tdx. What are my options or next steps to recovering everything?
Copy the old build-folder to another location and rename that folder to something else. The reason for this is to preserve this data in case you need to access it again at another time. Besides, it's always better to use copies of things when touching data.
The only way to see what's in this folder is to import this into Trainz, whatever version you used at the time or a newer version. At this point, I recommend using a new Trainz install rather than importing the content into your currently working version.
Instead of installing a fresh Trainz copy all over again, you can duplicate the program folder located in C:\Program Files\N3V Games. Open that folder location and copy the Trainz version and paste that elsewhere. If you have an additional hard drive, put it there. I recommend renaming that folder afterwards to something else such as TRS2019 - Test, or TRS22 - Test, depending upon which version it is. Once copied over, open the folder and right-click on TRS19.exe or TRS22.exe, depending upon which version this is, and choose Send to desktop (short-cut). This will put a shortcut on to your desktop.
With this version now copied over, create a new data-folder and DO NOT create it in the default Users\%profile%\App Data\N3V Games location. Create an empty folder on your second drive or on the same drive where you placed the program.
Start up your copied Trainz version. By default, it'll create a new database in the C:\Users\ area. Instead, click on the ... on the right and choose the new folder you created. Trainz will ask for graphics profile, you'll need to login, and you'll need to restart. This is a fresh copy of everything except for the program itself.
Once everything is set up, start this copy of Trainz again. This version will load very quickly since this is a very small data-set of built-in assets. Start Content Manager.
Choose Import content folder and let this version import that content from your old data-folder. Once this is completed, run a database repair to commit the content to your database. DLC most likely will show not-installed but other content should be fine. Once imported, you can now export assets to cdp files as needed to import the assets into your working version.
The method I outlined, other than importing the data folder, is similar to what we do for creating a test environment. The reason for a separate install is to allow for testing without touching what already works in case something should go terribly wrong.