TL;DR
You should be able to tell a train to leave a station if you have an amount of inventory equal to the current train's inventory, or equal to its filters.What ?
Currently when setting the "Item count" train condition, you can select a item and type a count.I propose adding 1 or 2 buttons to that dialog: "Current amount" and "Filtered Amount."
Clicking "Current Amount" would fill in the number with the current amount of that item on the train.
Clicking "Filtered Amount" would fill in the number with what the item inventory would be if all filtered slots for that item were full.
Alternatively (and possibly better from a user's point of view) a new condition, "Filtered item full"
This would check on the fly if all filtered slots of the type in the condition were full, so you don't have to specify a number (that may change as you modify the cargo filters on the train)