Penetrating continuous sound in C&O Hinton Division!

Neither, and both. Any asset from any source that you have downloaded after making a back-up copy of your whole trainzbase cannot be in that back-up copy. If you then restore the old back-up, it restores without those assets. If any such assets are still there, you`re probably doing your backup/restore operation incorrectly, which is an entirely different issue. Ok?
This is not a backup copy of the database.
On my iMac, backups are created continuously with Time Machine.
I have copied the database from such a backup.

Oooh, I have Back In Time on my Linux machine. (You do know that modern Macs run Linux under the hood, don`t you?)

Knowing how Back in Time works, and assuming that the relevant parts of Time Machine work similarly, be advised that any back-up program that operates on a file-by-file basis can leave in files that updated assets have deliberately removed. Trainz` own internal back-up system works on an asset-by-asset level, which handles orphaned files correctly.

On the other hand, most of the content is stored in *.tzarc files, which means that you probably got lucky, as those also represent an entire asset.

You are probably ok doing it this way, but be prepared for occasional oddball issues that no-one else can recreate. Of course, any time you mess with the trainzbase with external programs, you`re likely to require a rebuilding of the index.

Edit: If you just delete the entire asset directory before the restore, all of the points that I`ve been trying to make are moot.