How to supply multiple factories with specific amounts of materials from one train station?

Hey, so... I'm either over thinking this or I have a lot of rebuilding to do

A while back I set up a little plastic production factory - several refineries each making 20 plastic with a thought that I could use each refinery for future factories where plastic is needed. At the moment I have a train that picks up 20 plastic from first platform to the first freight car for "personal use" along with some rubber and takes it to warehouse.

Now I made computer factory that needs a little bit of plastic so I made a new train line that goes to the same plastic production station and picks up a few plastic but this time from second freight platform (locomotive - empty cart - freight cart) with a thought to pick up specific amount from second freight platform. Obviously this train also picks up the plastic from first freight platform - a fact which I somehow missed in the planning stage.

The question is in this case are the only options to make a new station for each new factory I want to supply from this base (not a fan of how bulky that would look)? Or make just one big train that picks up all plastic and delivers it all around the map.

I just wanted many trains in my world zooming around, coming to more of a central storage area, taking what they need and delivering goodies to whatever factories need them. :(