Can I...Divert train if station has cargo

[bear1]

Currently, a standard set up: I've trains running from a farm to a factory, returning empty. I'd like them to stop at a farm near the factory, fill up and drop that stuff off at a factory near the original farm. However, when I've done this in the past it falls down because the two farms produce different amounts.
Is there a way to tell a train to only go to Farm B and Factory B if the stations have cargo to collect?
I realise I could use the standard "Load any cargo" but I'd prefer them not to take the diversion if it's unneeded.

Thanks all

[odisseus]

Looks like what you want to build is a self-regulating network.