The trains won't ask for clay or products requiring clay and ore. At least I have never encountered that before.... Only planes and helis ask for them, but the planes can ask for a lot of clay. My advice to anyone is to never sell clay, cause why would you? It's barely any money and you have infinite space in your warehouse where it's stored. I have around 2000 on both platforms.

- But why?
- Why not?

I also buy these products on the market when I see them. Especially the ones that require ores, never produce them. Even after finishing with the upgrades, I will use the ores to produce jewelleries, not these items...