My own route started in Trainz04 long ago. it has been upgraded to all the subsequent iterations. Now it works near perfection in 19 (117009). Upgrade to SP5 and it shows 2,469 faulty dependencies in red. Most due to the Texture missing thing. I am able to use PVsoft conversion tool successfully on each asset with this error. But i will not fix 2K+ assets. It is a bad joke. Whatever gains I would get with SP5 are not worth the trouble. Same for 22. It looks like the regular 19 is the end of the line for me unless anyone has a quick solution. Probably this problem has been noted before, but I failed to see it...
Stop trying to fix the .texture problems they will be builtin or DLC assets that have some how been overwritten probably by also being in your local data.
Assuming you did a straight upgrade and did nor import into a fresh install, that doesn't sound right for an upgrade, try selecting all the faulty assets, right click revert to original, that may get rid of some of the errors if not all of them.
With Trainz after an upgrade to a new version never trust what you are seeing in content manager as it's invariably giving false information, never start trying to fix anything at this stage, you will make things worse in the long run
What has always worked for me is before attempting to upgrade, run a full Database Repair, that's press Ctrl while clicking Rebuild Database, the full repair gets rids of problems caused by caching as it deletes the cache and duplicated assets located in multiple places as well as rebuilding the database. This usually avoids false errors appearing but not always.
Then run the update, when it's finished however much tempted
do not open manage content,
do not start Trainz but run a database repair, this is in addition to the one that Trainz does itself which IMO doesn't actually do anything useful.
On completion you can now go to Manage Content.
Check open for edit, anything in there right click revert to original, stuff in open for edit after an upgrade is an indication that you have items in your local data that are now builtins or included in DLC and they will only have .texture files not TGA and Texture.txt, no need to attempt repairing them just revert to original.
Now check for faulty assets:
Try first checking errors and warnings, you can select them all at once for this. It may clear the errors however they may still show incorrectly as faulty, sometimes preview asset will clear the incorrectly showing errors, a bit tedious, or double clicking an asset will often clear it, or open for edit revert to original can be done in bulk similar or probably easier shut down Trainz restart and run a Database repair.
First run of any route after an upgrade will need the assets to be cached, need to wait for this to complete or you will initially have routes with things missing and not working giving the impression your install is broken, normally it isn't, need to wait!