I noticed something rather odd. I have a short track in a room adjoining my carpenter workshops that takes furniture and dumps down a hole into a furniture closet on the z below. Everything works great, stuff goes down the hole, it's infrequent enough that no haulers have gotten beds dropped on them yet, but I just issued a large order for wooden minecarts. There was already a steel minecart in the furniture closet, as well as a number of wheelbarrows. When 5 wooden minecarts were dumped down the hole, they did not fall. Looking at the hole with k shows 5 carts and open space. This doesn't prevent other items from being dumped through the carts, but when I assign vehicles from the h-v menu, they appear with a red X to the right of them, which I think means they are inaccessible, as no one would haul them. I sent a sacrificial hauler to go fetch the steel minecart. One cedar minecart fell, missing the hauler, but 4 are still floating.
I assume this is related to the stacking behavior toady mentioned, allowing us to make automatic minecart guns, with a vertical clip of carts over a roller or dwarf kicker (would this injure the dwarf? My hauler was fine, no injuries or combat reports). The games physics tries not to allow more than one minecart in a square, causing the carts to float above the lower cart, but the auto dumping forces the carts to pack together over the hole. I tried channeling the square above the hole and dumping another cart, to see if it would teleport up the stack, but no such luck.