NOT HOW But expiation of products, industry and rolling stock
Basically products are associated with industries (special track) and wagons or coaches. They are generated or consumed by industries at a given rate that normally can be set in surveyor.
The config.txt file has product queues which control which products (kuids) can be loaded on each wagon or coach. Passengers are just a special case of product by the way.
Wagons and coaches have special points on the mesh that mean something to Trainz, a.bog0 and a.bog1 for example are the bogey attachment points.
If a load attachment point (a.load0 a.load1 etc) is present on the mesh then it can be referenced in the config.txt file by one of the product queues to say that if the product is present then attach this kuid to that attachment point. This gives you a visible load. In general if the attachment point is not present on the mesh then you can't get a visible load. However it is possible to create a second mesh, referenced in the config.txt file, which has the attachment point on so you can get a visible load to appear on a wagon that does not have load attachment points.