By 1850, there were almost 300 coal mines in operation in the Ruhr area, in and around the central cities of Duisburg, Essen, Bochum and Dortmund. The coal was exported or processed in coking ovens into coke, used in blast furnaces, producing iron and steel. In this period the name "Ruhrgebiet" became common. Before the coal deposits along the Ruhr were exhausted, the mining industry moved northward to the Emscher and finally to the Lippe, drilling ever deeper mines as it went. Locks built at Mülheim on the Ruhr led to the expansion of Mülheim as a port. With the construction of the Cologne-Minden railway in the late 19th century, several iron works were built within the borders of the present-day city of Oberhausen.