The quarries, mines and works come under the control of the company that ran the Corby ironworks in 1930. They increased production and in 1933 the Church Mine and Church Mine North were connected underground. Henceforward the southern entrance close to the works was used.for both mines. They introduced diesel locomotives in the mines from 1933 except in the headings where horses were still used. In 1933 production was doubled over 1933. The ironworks continued production until 1942. During that time the works produced iron for casting, whereas the Corby works produced basic iron for steel making (from 1936 at the new Corby Steel Works.) In 1941 there was a proposal to convert the 3 foot gauge tramways to standard gauge and to connect them to the Corby works system to enable transport to those works without transhipment (except at the mine exit) or use of the main line railway route through Kettering. However this never happened. The quarries and mines at Slipton had closed by this time. The mine closed in 1947 and the Twywell quarries in 1948.The quarries at Lowick and Sudborough continued until 1952, still sending their ore by tramway past the village of Islip to the Islip sidings for transhipment to the main line. Calcined ore which had been left near to the ironworks was extracted until 1969.