What tier of items to ship by train

Hey folks,

I am in the process of building my first rail network and I am going back and forth what to actually produce on site and what to ship by train.

Example 1:

I have 4 pure iron nodes right next to each other (rocky desert) and I'm currently thinking whether to smelt on site and distribute the ingots over the map as needed or just produce one product like modular frames in large quantities there and ship them.

Example 2:

I'm also in need of automating computers, but uncertain whether to directly build computers at a pure copper node and ship in rubber or only make copper sheets on site and ship that out to another place where I assemble all parts into computers.

How do you deal with this. What tier of items do you send by train and do I get issues with too many trains when I send too many smaller parts?

Ty for the input!

P.S. I dont want to use trucks since they require fuel and I hate dealing with that. Havent unlocked drones yet.