gwhess - But wait, there's more!
When you are setting up which commodities will be carried by a particular wagon, use the F7 key in Surveyor, Session Edit mode. Choose (select) the wagon/ hopper that you wish to load.
If it is empty and does not show any particular commodity/ product, then choose the Big Plus (+) sign to choose the desired product (virtual 'load'). Make sure it is appropriate for the type of vehicle. (Liquid/ vs solid, etc.)
Asset creators will list the available (appropriate) commodity load-types in the 'Allowed products' section of the Config.txt for their vehicles, so you can check the KUIDs for Load0, 1, 2, 3, etc. allowed there. (Content Manager, Select the wagon, Choose Open... Edit config.txt to view the file). Sometimes, the loading operation and the fully laden vehicle will be animated/ show the product to be carried/ loaded, but not always. Many products/ 'loads' are invisible.