Yards such as Shire Oaks and Newell are the staging areas for coal trains out of Southwest Pennsylvania. Coal trains start when the Shipper (Bailey or some other mine) gives the carrier an order for a train to load. The order can specify rotary dump bathtub gons, open top bottom dump hoppers, a private owner train set or a blanket number of cars of any open top loading kind. The support yard assembles the train that the Shipper wants and a crew with road power will take the train to the mine, load it and return the loaded train to the support yard. Road crews will be called from the support yard to start moving the loaded train to its ultimate destination - the destination may be a Export Coal Pier or a domestic industry or power plant. At destination the trains will be emptied and the cars returned to the support yard for future loading. The 'Coal Department' of the line haul carriers are the ones that supervise the movements of coal and empties on their carriers. As a young adult I lived in Bethel Park and used to go out for drives around the area in my Triumph TR-4a, often passing the Elrama Power Plant - late 1960's
Neat! One update is the rotary coupler end is identified by any contrasting color. I have seen cars with baby blue, white, yellow, red, orange, mint green and others. Here's a great Flickr album of early gons and hoppers for other examples. https://www.flickr.com/photos/63288316@N06/albums/72157669125810681
In most case of coal going to power plants, the cars are owned by the electric utility. Because the utility is supplying the cars, they get a rate break over what they would be paying if the railroads were the one supplying the cars. Most utilities have more than a single set of equipment that they own. In some cases, a set of equipment may get held at the power plant so that the utility's maintenance contractor can fully inspect and make the necessary repairs to keep that set of equipment rolling for its next hundred thousand miles or more.
