There are two primary types of gondolas: Mill gondolas that haul finished steel products and scrap steel and coal gondolas that run in unit trains from the coal mines to utility power plants.
Coal is often carried in an open-topped car called a gondola like this one. Coal gondolas have rotary couplers and when it's time to unload, they can be rotated by a rotary dumper to allow the coal to pour out into a pile or haul-away pit. This can be done without uncoupling the car from the rest of the train. We also use rapid-discharge open hoppers that unload coal from the bottom by use of pneumatic gates on each car.