Minecraft wiki entry for Flying_machines, associated text: Driveable Machine C is a 2-way machine which can be reconfigured in the field to go the other two ways, by mining and replacing four blocks. It can carry two passengers, and has two cargo bays for minecarts with chests (or plain minecarts, for passengers). It uses 20 slime blocks including the cargo bays. Its use is complex, but may be worthwhile just for bragging rights.