Tacoma Rail is a publicly owned Class III shortline railroad. It is owned by the city of Tacoma, Washington and operated as a public utility. It is one of three operating divisions of the municipally-owned Tacoma Public Utilities service, but unlike other city services, the railroad is self-supported and generates revenue for the City of Tacoma and Washington state. Tacoma Rail provides freight switching services, serving the Port of Tacoma and customers in Tacoma, south Pierce County and parts of Thurston County. It operates 16 diesel locomotives, more than 100 employees and about 140 miles (230 km) of track, many of which are former Milwaukee Road and BNSF Railway lines around Western Washington.

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.