The method used to unload coal depends on the plant and when it was built. Up through the 1960's coal was unloaded through traditional 2,3 and 4 bay hoppers into pits under the track. The doors on those cars have manual locks, that can be opened with a pry bar. A small crew of men were required to unlock and dump the cars, and then to swing the doors closed again and lock them. It took about 24 hours or more to unloaded a train this way. In the 1970's rotary dumping came into vogue. Rotary dumpers had been around since the 1920's, but those operations require each car to be dumped on it's own. What made it popular in the 70's was the use of unit trains of gons equipped with rotary couplers. Now the train could be kept intact, and it only took 12 hours to unload a train. In the last several years, 5 bay rapid discharge cars have come to be the new standard for new or rebuilt installations. These cars have a automated door system powered by a air cylinder on the end of the car, air is taken from the Main Reservoir of the locomotives. By using a these cars on a elongated pit, the train never has to stop (ideally) while unloading. The cars unload at a steady .5mph or so and a train can be turned around in less than 6 hours. I worked as a locomotive engineer at a plant with these Rapid Discharge cars, and it is the way to go when it comes to unloading coal. MDC made a early version of the Rapid Discharge cars. Walthers makes 6 car sets of the Trinity RD4 cars, Exactrail did some Johnstown America RD cars in 4 packs. The Walthers cars are the most typical kind of car, as the newer JA cars look similar.