The NRE 3GS21B is a low-emissions diesel genset locomotive built by National Railway Equipment (NRE). Instead of a single prime mover, the NRE genset locomotive is powered by three separate 700 hp Cummins QSK19C engines providing a total power output of 2100 hp. Each engine could be individually started or shut off depending on how much power is needed, reducing overall diesel emissions and improving fuel efficiency. More than 150 of the 3GS21B genset locomotives have been produced to date, with the majority of these units being manufactured at NRE's Mount Vernon shops in Southern Illinois. In addition, three road slug models have also been produced. The main difference between this and the NRE 3GS21C is that these units are on B-B (four wheel) trucks and the others are on C-C (six wheel) trucks.

Flexibility

Because the engines are modular, flexibility is obtained:

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