Minecraft wiki entry for Hopper, associated text: Minecart with Chest
Minecart with Hopper
Unlocked hoppers fill chest minecarts and hopper minecarts if any part of the entity's hitbox is within the hopper's target block-space. Hopper minecarts try to pull items from the hopper at high speed. Hoppers can pull items from minecarts above them so rails can be placed directly on the top faces of a hoppers. If a detector rail is in the right position, it could lock the hopper per standard redstone-hopper behavior.