It sound complicated :D Have a look at this system I created. Setup is EZ like hell.
All stations that require certain goods need to be called the same. Also all the ones that provide certain goods need to be called the same. On any station all u have to set is trigger amount of staff and signal ...

Ok I made it Many to Many improvement. Heres really easy setup https://imgur.com/a/7AGA5Zs and BP https://pastebin.com/Ybr8hjNq

I thought about it for some time now, and I simply cant see how you can make many to many system without trains waiting to fulfill orders one by one. No information can be sent to train about which station he is to go to so the only way to limit it would be to not send more than one train at a time ...

Yes one train one route system. But since the stations are getting turned off when they are not needed u can easily set it up your way too.
- Use same stations blueprint as providers set conditions like so http://prntscr.com/s8icia
- Set route like so http://prntscr.com/s8iee8

You get the same ...

All info and files are uploaded to https://factorioprints.com/view/-M65XHOd6ky27QlzbWeQ

My idea was to make one huge parking depo from which any requester station can pull a train when it needs to. I wanted to use as little resources as possible, including one signal wire if possible. It seems to ...