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A term originally used by railroads. Before the use of automatic signals, a large ball on a string hung over the tracks was used. If it was low, the tracks ahead were possibly occupied and a low speed was supposed to be used. If the ball was raised high, the engineer could take the train to full speed because the track ahead was not occupied by another train. Out of use since the early 1900's or earlier in many places.

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14y ago

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