I'm glad you found this useful.
Missing content may mean going on a kuid hunt as we call it. Using a forum post called Missing Assets? Post them here, you can post the list of missing assets by highlighting them in Content Manager, then copying and pasting the list in the forum where someone will assist you with finding the assets.
Mods, please make this post sticky. I've been inspired by threads over on the SI3D forums for missing kuids post, and thought that the Trainz Forums should have a similar post. If you have missing kuids, post them here, and respective users should know what or where to get the kuid. You will...
forums.auran.com
The alternative is to do search in Bing or Google. Put in the kuid in the search box prefixed by Trainz kuid. This will bring up possible links for it as well as mentions of it.
In the event that all else fails, you may as well as forget them and edit the route, and then delete the missing assets. This is something of a last resort, but it has to be done even if you find replacements in order to clean up the old ones from the route. In many instances, the missing assets are something you can live without such as a tree, or building, or some other insignificant asset. Other times, you need to replace them manually for such things as switch levers, bridges, tracks and other more important things.
Faulty assets can be a time-consuming challenge. It takes experience and knowing your way around as well as understanding the error messages. At this point in the game, I can repair most things except for script errors. but it took me a very long time getting here.
The easy errors to repair are spelling errors in the config.txt files, missing textures, and incorrectly sized textures. Over the years, I've gathered what I call a toolbox for repairing assets. This is like having a scrap box of extra bits and parts when building a model or model railroad. In that folder are miscellaneous textures along with their texture.txt files, such as the famous black.tga and black.texture.txt, variations on the always missing screenshot.jpg files for those quick missing thumbnail errors, and many other config.txt parts for such things as missing or faulty mesh-tables, etc.
Your tools should include a text editor such as Notepad++, but Notepad from Windows will do the job. Helpful tools include Search and Replace Master, and a payware application File Boss. Search and Replace Master allows you to repair multiple assets with the same typographical errors in the config.txt files. This has to be done in multiple passes, but it beats opening up hundreds of similar assets to fix the same problem. File Boss can copy the same file to multiple folders at once. This is extremely useful for those times when the same texture is missing from multiple assets.
Other tools such as Images2TGA, and PM2IM are part of this toolbox as well. These allow you to decompress .texture files to the two parts - texture and texture.txt, and convert or update meshes from the old PM to the newer IM meshes respectively.
As I said, this takes experience but above all it also takes patience. Error repairs don't always go as expected and an attempt to update or repair an asset can lead to further problems. Sometimes, it's necessary to download the asset again and attempt the repair from scratch.
Your best bet initially when asking about repairing an asset is to start a thread and ask for help on it. We can walk you through the steps for repairing the assets. Eventually, you'll gain enough experience with these errors to be able to take them on yourself.