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 horsepower (522 kW) Cummins QSK19C engines providing a total power output of 2,100 horsepower (1,570 kW). Each engine could be individually started or shut off depending on load reducing overall diesel emissions and improving 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 NRE's Mount Vernon shops in Southern Illinois.^[2] In addition, three road slug models have also been produced.

Because the engines are modular, flexibility is obtained:

• If one engine fails, the others can continue at reduced power.
• If a lower axle load is needed, one engine can be left off.
• When reduced power is needed, one or more of the engines can be turned off, saving fuel and wear and tear.
• The QSK19C engine is common to the four variants of the NRE 3GS21C locomotive.
• The QSK19C engine is also widely used for non-railway applications, and spare parts are readily available.^[3]

• A brand new 3GS21B locomotive painted for the BNSF Railway.
• An NRE 3GS21B leads a Union Pacific train along street running trackage at Anaheim, California.
• Two brand new U.S. Army 3GS21B locomotives working at Fort Lewis, Washington.
• Brand new MBTA 3GS21B at South Station.
• MBTA 3GS21B on the Grand Junction Railroad.

• NRE 3GS21C