Minecraft wiki entry for Redstone_components, associated text: A minecart with hopper pulls items from containers above it and push items into hoppers below it (the number of items transferred can depend on how long its velocity allows it to remain within reach of the containers). It also picks up items that have fallen on the rails. If a minecart with hopper passes over a powered activator rail, it stops transferring items indefinitely until it passes over an unpowered activator rail.