More than 1100 class SM42 locomotives originally built by Fablok as type 6D for PKP between 1963 and 1993.

PKP IC say the locomotives will be rebuilt with modern diesel engines meeting EU Stage 3B emission standards whilst reducing noise and maintenance costs. Microprocessor control of the engine and traction systems will improve performance and reduce fuel consumption.

While primarily used for shunting empty stock at major stations, the locomotives will be equipped with electric train supply, enabling them to be used to operate seasonal passenger trains on non-electrified parts of the Polish network, serving resorts on the Baltic coast and in the Carpathian mountains in southern Poland. These services are currently mainly operated using the unrebuilt class SM42s remaining in the PKP IC fleet.

In 2014/15 Newag rebuilt 20 SM42 locomotives for PKP IC in two batches: 10 equipped with electric train supply for passenger trains (as class SU42.10) and 10 designed as shunting locomotives (class SM42.30). Large numbers of SM42s have been rebuilt by various manufacturers for use by freight operators in recent years.