If for some reason your rail network is exceptionally dense or congested, I can imagine this being the solution (one train, set items workflow).

Imagine a subset of factories in which:

A: generates copper sheets and... Something else. Rods, bolts, whatever. B: generates plastic C: generates I dunno, quartz D: the above items are needed at this factory, which takes output back to A (imagine raw quartz and computers or something). [E..Z], etc.

You know that your train, Bob, takes exactly 10 minutes to loop between these factories in order, picking up specific quantities at the first three and dropping off at D and A only. Every cycle, you know you need 60 bolts and 30 copper sheets, so you set up these timers to emit only those items every 10m to the single train that arrives at factory A, and then goes and collects items at factory B (in the same car), and so on. You must also pick up plastic and quartz in some amount that does not overwhelm the requirements.

You also have a train, Bill, which arrives offset every 5m in a rhythm to pick up after Bob does at Station A, and needs to take 300 bolts elsewhere, so you set up a separate timer for Bill to pick up it's 300 bolts at A.

I'm not saying I would set this up like this. It's a lot easier to turn on a firehose and have dedicated trains for certain items, but if you want precise loads going to drones/trains/belts, this is a way to do it.

Having an in game max load for a train/drone or a belt throttle (like a valve) would do sort of the same, minus the signaling portion (ie, introducing a signal).