Historically the mix of coal and rock was picked over by hand to separate the valuable
coal from the unwanted rock and the rock was then dumped on a pit heap.
Hand picking was inefficient and a lot of small coal (under 30mm) ended up in the pit heap.
Modern coal cleaners use the differential in specific gravity to separate coal and rock.
After their development it was realised that small coal could be obtained by
digging up pit heaps and running them through a modern coal cleaner.
Small coal is used for industrial purposes such as in brick and cement making.
On the layout a pit heap is just starting to be dug to obtain the small coal.
The narrow gauge moves the coal and rock to the cleaner where the coal is
separated and loaded into wagons and the rock returned to the pit heap.