A narrow gauge railroad line between Sewell and Rancagua was started in 1907 and fully in use by 1911. This became the only way in and out of the mining camp for personnel and goods. The track was protected by wooden sheds to keep the snow away.
World War I created a huge demand for copper and the mine responded. It grew to hold 14,000 inhabitants in 1918 with an accompanying growth of housing, social services, entertainments and cultural facilities. In 1921, smelting operations moved to Caletones, about 4 miles (6 km) downhill.