Using F3 to take you down to the smelting area, there are two hauling routes. Route 1 has had a workaround implemented, where if it's full of desired items it leaves immediately otherwise it waits 10 days and leaves if it's at least 50% full.

Route 2 shows the original state and what triggered the bug, as due to the bins within the minecart (as determined when I asked for help in the below thread.
[url]http://http://www.bay12forums.com/smf/index.php?topic=143391.0[/url]

Basically, because of the bins in the minecarts, even though they are full, they aren't full of the "desired" items, therefore meaning they won't be hauled anywhere.

Just noting here that the save comes from the Starter Pack r4, just in case it's needed.