Does anyone else do this? It's far from a fool proof solution, but for items I find and want to download directly myself I click on "Download Details", then "Download FTP", copy and append the name of the item to the front of the kuid and save to a separate folder as an archive. Not the dependencies, just the item I want. That way I have the item and a description saved for reference. The item can then be identified and installed manually as necessary, with any dependencies installed after the fact via Content Manager. Yes, its a bit anal and redundant, but I still prefer maintaining my own backup of individual items. (One of my major peeves with the system as is, is the complete disconnect between a kuid and its description until installed.) And yes, it requires some extra disk storage (which is really cheap now) - but so far it's worked nicely for me and saves me from having to remember the appropriate search criteria to find an item for a newer Trainz version. Or for items that were used in an older version, but still listed for a later version that are now vaporware. (I was an "on again", "off again" user for years until I retired so continuity between versions was not always maintained. And I prefer trying to maintain a single game version vs multiple versions as some do - less complicated in my view, but that's just me.)
Whether or not that helps versus just doing a backup of the Local Data Folder is your decision. That is likely the easiest solution, but I'd like to think I have more control over the items that I want to keep. I'm an old IT guy, so what do I know. I still get irritated that I can't delete obsoleted payware/built-in items.