Here's my latest iteration of the design:
It works something like this:
[*]The signals U and L mean Unloading and Loading respectively.
[*]The top-left combinator sets U=1 if any f the train's order signals are negative.
[*]The bottom-left combinator sets L = 1 - U.
[*]The top-right combinator subtracts the train's contents from the train's orders.
[*]The bottom-right combinator multiplies each of those subtracted signals by L (ie if the train is unloading then they all come out as zero).
[*]The miniloaders flowing out of hte bottom of the warehouse are filter miniloaders, the their filters set by the signals coming out of the bottom-right combinator. So if L=1, the filters on these miniloaders are set to the items which the train is still requesting.
[*] The miniloaders flowing out of the right-hand side of the wagon are enabled if U > 0.
So if a train is delivering, U=1 and L=0. The miniloaders flowing out of the warehouse are disabled and the ones flowing out of the wagon are enabled. If a train is loading, U=0 and L=1. The miniloaders flowing out of the warehouse have their filters set to whatever the train needs to collect and the ones flowing out of the carriage are disabled.
Now, this almost works. It mostly does work for deliveries. A train arrives, U=1 and the contents of the wagon get unloaded into the warehouse. For collections, though, it's a bit more difficult. It has a tendency to slightly overload the wagon; at this point the input to the top-left combinator goes negative, then U=1 and it starts unloading again. It sits a cycles through this pattern until the two-minute expiry is up; if I'm lucky, the train will be unloading when it leaves and everything more or less works out. If I'm unlucky, the train will be loading when it leaves. This leaves a big pile of whatever it was loading on the belts to the right of the wagon and the next train to arrive will pick them up.
Has anyone got a better design for stations that both load and unload? Moving the warehouse closer to the wagon helps in some ways as it reduces the number of items that can be buffered on the belts, but unless you have a single inserter with a stack size of 1 doing the loading and unloading, this is always going to be a problem at some level. I guess reading the contents of all the belts and miniloaders, as well as the warehouse, would also probably fix it, but it would be both a terrible hassle to set up and rather ugly. I guess I could also introduce some sort of hysteresis using combinator latches, but wanted to ask if anyone had better ideas before I added that much complexity.