The 3GS21B is a genset locomotive offered by National Railway Equipment. Power comes from three Cummins QSK19C with 700 hp each, which are standard engines widely used in the industry. In contrast to the 3GS21C, it only stands on two-axle trucks. From a total of more than 180 built, BNSF was the biggest customer with 72. Also the Union Pacific ordered 59. Additionally to the standard locomotives, three slugs were built without cabs and diesel engines. 