It is a combination of reasons. Some are old and no longer on the DLS or whatever 3d party site they once were on. Some are built in assets in TS12. Most recent content created to date has been built for 2009 and above. However some of the needed kuids are only found in TS12. I downloaded several steam locos/tenders the other night and most were missing as many as 7 unknown assets. As an example, one loco was missing four version :3 assets. Example: a bogey (kuid2:96914:504770:3). I found this asset in my 2006 version as 96914:504770 as well as the other three. The :3 MAY be a built-in in TS12, I'm not sure. But, I exported the "lower version" assets from 2006 in 2010 via a CDP, updated the loco's config file, and that solved the problem. I had a total of 15 unknown kuids in several locos/tenders and solved 12 of the 15 using things from 2006. I did substitutions (or deleted) the other 3. For example, the unknown kuid in one was an interior (kuid:523:1972918) for which I substituted 523:55901 that is in 2010. Trainz has always been sort of a "hide and seek" game when it comes to finding assets and their components. There is a lot of kuids you cannot find via Google or the Trainzkuidindex.
