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A rotary car dumper or wagon tippler (UK) is a mechanism used for unloading certain railroad cars such as hopper cars, gondolas or mine cars (tipplers, UK). It holds the rail car to a section of track and then rotates the track and car together to dump out the contents. Used with gondola cars, it is making open hopper cars obsolete. Because hopper cars require sloped chutes in order to direct the contents to the bottom dump doors (hatches) for unloading, gondola cars allow cars to be lower, thus lowering their center of gravity, while carrying the same gross rail load. Cars are equipped with rotary couplers to allow dumping them while they are still coupled; a "Double Rotary" gondola or hopper has rotary couplers on both ends to allow it to be unloaded while it remains coupled to stationary cars at each end.