Atlas HO scale unit train gondola

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Atlas HO scale unit train gondola

Is there some Manufacturer that builds gondolas which could really works? I mean that you can really discharge the freght car from the bottom? I’m trying to build a working coal layout and I really want to load and discharge black sand (coal) into/from wagons…or the only chance is to build a system to turn wagons 120° as in Norfolk coal Terminal?
Many thanks ModelRailroader!