The English-owned Nitrate Railways served the mining areas along the Tamarugal plain, an inland strip parallel to the coast about 50 km inland from Pisagua and Iquique. There were many short railway branches, some owned by the mines, that ran between the mining offices and the towns. Some were animal-powered: a common sight in the early part of this century was the little mule train transporting workers between a residence camp and a railroad station. Such a line connected Paposo mine, owned by W. R. Grace & Co. of the USA
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The GR is for the Poposi mine.