Re: NYC 499680 flat and other flat cars at Joy Manufacturing Co (undated)
It is now simply known as "Joy", a part of the Komatsu America line of mining equipment. Joy absorbed P&H Mining Equipment via the P&H bankruptcy sometime in the 2013 - 2015 era, and became "Joy Global", but then got bought by Komatsu in 2018. Milwaukee is the HQ for Komatsu America.
There are still plenty of "Joy" facilities around. Here are a few associated with the Appalachia coal fields:
The machines on the flatcars at that plant appear to be "face loaders" (on the front two flatcars) and "shuttle cars" (on the back four(?) flat cars against the building). It appears there is a separate 'mine truck' type vehicle on the ground between the building and the track with the shuttle cars, or maybe there is an additional track there.
The loaders have a pan type "apron" that is driven into the broken coal at a working face, and two gathering arms that pull the coal into the center of the pan/apron. A conveyor then takes the coal up and over the machine, and dumps it into a waiting "shuttle car".
The "shuttle car" then transports the broken coal to a large storage bin, where it is unloaded, and subsequently loaded into a mine train for hauling out to the surface, or the bin might be connected to a shaft, where the coal is hoisted to the surface. The shuttle cars simply "shuttle" back and forth from the face to the bin. They have a small conveyor in their bottom to move the coal out of their bed hopper. Shuttle cars do not 'dump' in the conventional sense of tipping at an angle to let material slide out the back.
The stuff on the flatcars is very similar to the modern underground machines. Here is a link: