Simple-to-setup general train resupply for outposts. May undersupply depending on where the items are needed, because they may not be removed from the requester chest by bots quickly enough. May also oversupply (which I think is good, given the former problem), because an item will keep being moved to the active provider chest until logistics bots move it to the requester chest. This is not slick or precise, but it will get the job done eventually, and it’s simple.
At the dropoff station: Use the constant combinator to set the stock you always want to maintain as a buffer. The decider combinators next to the roboport set how many robots the outpost should be stocked with. The selector combinator should have its output signal set to a signal that is used in the train stop name. All outposts to be resupplied by a given train must have the same format for their name, except for that one signal.
At the pickup station: the selector combinator gets the signals from the radar network, and sends the highest one to the train via the train stop. The size of the signal is just how many different items are under their desired stock. The requester chests keep stock for the train on hand, and the combinators set what the inserters will move so that the train is filled with the sum of all the things the requesters are asking for. Need to make sure this fits all on one train car. If you need more space, you’ll need to duplicate the setup at both stations.
Train settings: its only stop should be the pickup station. Then an interrupt can be set when you get a signal from the train stop to go to the stop that has that signal in its name.