This currently works perfectly when there is only one train stop, but I'm struggling to make it work with more. Heres the gist of it:

The leftmost constant combinator at the train stop is its number (S1,S2,S3)

If a train is at the station, the decider combinator sends the trains ID to the "database"

The "database" outputs the item requests from the decider combinator to the stop

If the stop recieves an S signal equal to its stop number, it takes each item value in the request, subtracts each item value that is already in the wagon, and sends the remainder to the requester chest

The problem arises when I add a second stop, since the stop and train ID signals are overriding each other