It sounds like something has gone wrong with your Trainz installation - the asset 'General Goods' is a built-in asset with a negative KUID number (which basically means it was made by Auran).
The kuid is <kuid:-3:10013> - if you can find it in Content Manager, right-click it, select Edit, then select Revert to Original and see if that solves the problem.
If it does not solve the problem, you may have to reinstall Trainz and any appropriate Service Packs (If it is TRS2006, SP1 has to be installed before running the game or CMP for the first time after installation)
Shane