Quite allot of all assets have been upgraded to trainz-build 2.9, that were once 04, 06 trainz-builds.
These assets, and all of their dependencies, must all be examined in the CM by "View Config" file. Just one dependency will stop an asset from showing up.
If they are higher than your trainz-build, the trainz-build numeral must be meticulously manually backdated to be your trainz-build , or a lower trainz-build numeral. A lower trainz-build numeral need not be edited, just leave them be, as they are.
To find your trainz-build, make a "New Route" and examine the routes config file trainz-build numeral, that is your trainz-build.
So you edit the numeral: 3.3, 3.0, 2.9, 2.6, 2.5 ... etc ... to be your trainz-build numeral ... but trainz will still see the original higher trainz-build, until you make the TAD rebuild itself.
In TRS2006 (possibly the same in TRS2004), the 2 files: assets.bku (in cache folder) ... and assets.tdx (in TRS2006 folder) must be deleted to your PC recycle bin ... then restart the CM, and let it rebuild for @ 20 min or longer and don't use your PC: Go warsh' da' dish's, or go outside and do a Dawg Dud Recon, Search & Rescue Mission (Motto: No Dud Left Behind), or vaaccuumm the house, vaaccuumm the Kat ... do not vaaccuumm the budgie ... "Zooook" !
Some assets once edited will show errors and faults, and these can be fixed, by manually editing config files (PM me, or get on Free Skype, for help with problematic assets)