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1) A specific type of model electric train. Known both as H.O. scale and H.O. gauge. The letters come from "Half-O", since H.O. was originally half the size of an O gauge train. The scale of H.O. is 1/67, and its gauge is 16.5mm. H.O. trains are the most popular size in the world. 2) the scale of H.O. trains, which is 1/87. It can eb applied to trains, minitaure figures, or model kits.
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