The outposts are sending a list of items they require via a radar. This can be based on logistic network requests, or constant combinators or whatever tickles your fancy.
The radars are transmitting the items they need encoded as bitmask, so one channel can support up to 32 outposts. At the train station, I am matching the ID of the train to an outpost, look at what the outpost is requesting via radar and then set the requests of the chests to the required items. I'm using a selector combinator to always request 2 stacks of every required item.