Before the beginning of technological advancement in the coal industry, the coal used to be loaded by hand onto mule drawn carts and taken out of the mine.  As technology developed, they began using conveyor belts to exit coal from the mine.  Below is an image of one of these conveyors:

    For a time this method was sufficient, but as the mine grew farther into the hill, it became increasingly difficult to extend the conveyors to the mining walls.  The need for a new mode of transportation arose, and below are pictures of three different solutions to the problem: