What
When an artillery wagon is partially full, the Train GUI states that it has "Full Cargo" (indicated by the solid green wait condition).When an artillery wagon is partially empty, the Train GUI does not show any progress indicator that it is working towards "Empty Cargo".
Expected behavior
Since the artillery wagon has an upper limit of 100 shells, I would expect the progress indicator of the wait condition to increase as the upper/lower limit was reached, as it does for all regular wagons.
Edit - Extra image for comparison
Edit 2 - Added Cargo Empty information
