This is not an uncommon problem and has been the subject of several forum threads. The cause is the presence of at least one commodity product that you have downloaded that is faulty (but may not be showing up as faulty).
Firstly, use CM to look for and delete all commodities that are faulty
Category = commodity
AND
Faulty = true
If the problem still persists, then the more drastic solution is to delete ALL the commodites that you have downloaded from the DLS - those that came with the Trainz install (i.e. "built in") can be left alone.
Category = commodity
AND
Installed = true
AND
Builtin = false
I had to resort to the later solution to solve the problem. A few of my routes then displayed missing assets, a few commodities, that were on the DLS. Since it was possible that one of these was the cause of the problem, I downloaded them a few at a time and after each download I started Surveyor and tested the products list. I found no problems and concluded that one of the deleted and now unneeded commodities was the problem.
