Cargo Loaders and Cargo Unloaders are functional blocks in Minecraft. They are triggered with a train passing over them similar to the Announcer block and the Redstone Sensor block.

The Cargo Loader works by placing down a chest on top of the cargo loader.

When a train carriage passes by the cargo loader it'll take 1 item from the chest and carries it until it touches a cargo unloader.

← Image of a Cargo Loader.

← An example of a chest on top of the cargo loader.

The chest can not be under the cargo loader since in Minecraft, you can't open a chest if there's a solid block on top unless it's a slab.