I tried a couple of other people's designs for this, but I quickly discovered that they were overcomplicated with sorting combinators or they were attached to the logistics network. That causes a problem when you have a large or very long wall outpost that needs multiple train stops for supplies and ammo instead of limiting it to only one. When you have multiple train stops connected to the logistics network instead of a local chest, the train will get confused as to which station is requesting which items. Isolating the requested item logistics to the chest itself ensures that the specific station needing the items gets what it needs. This isn't a new idea, just a simplified version.
The idea is to use an item Logistics Group that can be maintained in one place to define what supplies are requested at every outpost. The moment an item is removed from the requester chest, the supply train is dispatched to refill only what's missing from the Logistics Group item list. Currently, there is only one chest used, but you could modify it to use multiple chests. With legendary chests, I find that I never needed more than one for my outpost supplies.
NOTES:
If the train cargo wagon doesn't contain all items in the list, it won't leave the station.
The inserter at the provider station is a stack inserter with default settings (no stack size override), which means it will insert slightly more than requested at times. However, this is preferable to inserting only one item at a time.
