For its most difficult section, the Nitrate Railways ordered six heavy tank locomotives from the Yorkshire Engine Co. in 1922. This section combined a grade of four percent with curves with a radius of 280 feet. The locomotives had the wheel arrangement 4-8-4T, but a rigid wheelbase of only twelve feet thanks to their small drivers with a diameter of only three and a half feet. For additional safety when running downhill, they were equipped with a Le Chatelier counter-pressure brake. The bunker behind the cab held 1,000 imperial gallons of oil, but was prepared to be converted to coal if needed.