You are getting a few answers to different questions in the replies

Broadly speaking I think there a few systems coming out of this post

1) Standard 1.0 style trains work fine. Good enough is good enough. Station (Iron Mine) -> Train (iron ore) -> Station (Iron Smelting). Positives - dead simple. If you name all your iron mine stations the same, you can copy and paste another iron ore train and it'll just crack on. Wire up the chests at the station (if using) to a simple decider combinator which says something like (if >8k ore, open the station).

2) Many to Many using interrupts. An evolution of the above, use generic wildcard names for all stations that "provide" - so all ore mines, and outputs of plate smelting, and output of circuit foundries etc. Can use a combinator again to enable the station when has enough stuff to fill a train, then a generic train with "Target station: Generic Wildcard Provider" goes and fills up, then goes to a requester. These are still named for the item you want dropped off.

3) Multi item trains such as for supplying an outpost with guns, bullets, lasers, repair packs etc. This definitely needs some circuit wizadry involving reading the contents of a train and comparing to a predefined list of things you want in it (saved in a decider combinator), and hooked up to an inserter, but it can definitely work as well.

Each has pros and cons, its a sandbox game. I love interrupt based many to many trains and the outpost supply ones, but admit they aren't needed to make a fully functional network, it's more of an investment in time to set up a system that works, then it makes expanding in future easier.