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.

Cargo Loader
Render of the Cargo Loader block.1)
Renewable	Yes
Stackable	Yes (64)
Tool	From Stone Pickaxe
Blast resistance	TBC

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 for every Minecraft Tick that the cargo loader is right below the train's head from the chest and carries it until it touches a cargo unloader.

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.

The Cargo Unloader functions nearly the same as the Cargo Loader but the opposite way.

When a train carriage passes by the cargo unloader it'll put 1 item for every Minecraft Tick that the cargo unloader is right below the train's head back into the chest next to the cargo unloader, leaving the carriage empty.

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

As mentioned previously, the cargo loader/unloader loading is not instantaneous and works by transferring 1 item for every Minecraft tick.

If you would like to load all items from the chest, you should consider putting it at the front of the platform, that way the train can slowly transfer the items during the boarding period.