Which wagon are you referring to? You got a picture?
The main wagon mostly uses the ingame filters rather than a custom UI, I guess I could sort the filters but easy enough for a player to do so when configuring them.
Or do you mean the other wagons when it is pushing out trash items? That has more potential, not only to sort the contents of each wagon but which wagon it decides to drop items into. Certainly as an option if can decide the exact rules.