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. 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 San Diego and Imperial Valley Railroad (SD&IV) is a class III railroad operating freight rail service in the San Diego area, providing service to customers in the region and moving railcars between the end of the BNSF Railway in Downtown San Diego and the Mexico–United States border in San Ysidro. The railroad has exclusive trackage rights to operate over tracks of the San Diego and Arizona Eastern Railway, a subsidiary of the Metropolitan Transit System, the regional public transit service provider. Tracks are shared with the San Diego Trolley, another subsidiary of the Metropolitan Transit System, and freight trains are only operated at night when passenger service is not in operation. The San Diego & Imperial Valley Railroad was established in October 1984 and is owned and operated by Genesee & Wyoming, a holding company that operates more than 100 shortline railroads like the SD&IV.