The NRE 3GS21B is a low-emissions diesel switcher locomotive built by National Railway Equipment. It is powered by three Cummins QSK19C I6 engines with each one developing 700 horsepower (522 kW) and creating a total power output of 2,100 horsepower (1,570 kW). Having multiple engines instead of a single prime mover, a design referred to as a "Genset," reduces overall diesel emissions and improves fuel efficiency.[1] More than 150 of the 3GS21B Genset locomotives have been produced to date, with the majority of these units being manufactured at NREC's Mount Vernon shops in Southern Illinois.[2] In addition, one road slug model has also been produced.