Since the MÁV had already had good experiences with the Mallets of the type VIm (or class 651), even more powerful Mallets were built as class 601 from 1914 onwards. Thanks to an additional leading axle, they now had the wheel arrangement 2-6-6-0 and were considered the most powerful steam locomotives in Europe at the time. Their job was now to pull freight trains across the Croatian Karst without helper locomotives.
The 57 locomotives built were split among Yugoslavia, Romania and Czechoslovakia after the First World War. Due to an outstanding order, the MÁV received three more in 1920. The Oriental Railway also ordered three identical locomotives in 1918, bringing the total to 63. All operators retired their locomotives in the forties and fifties. 