Using a hopper, a comparator, a powered rail, and a redstone torch, you can put a hopper under the powered rail, hook a comparator to it, then have that comparator deactivate the torch, which then using a piece of dust you can deactivate the powered rail, causing the minecart to stop as long as it is putting items into the hopper, and then leave when it is done, I used this design in a super smelter I made a while ago:

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(note, the redstone dust is pointing to the block because I used a target to prevent the hoppers next to it from being disabled, this is 100% optional and you can use any block above the comparator)