The SM31, called type 411D by its manufacturer Fablok, is a six-axle diesel locomotive designed for heavy shunting and transfer service. Based on the four-axle SM42, it has a more powerful variant of the same engine with 1,200 instead of 800 hp. After the first two prototypes were bought by industrial operators, 167 locomotives were delivered to PKP until 1985. 28 more were directly built for industrial operators and designated Ls1200. Today, the last operational ones are still being used for shunting and freight trains over short distances. 