The MPI MP21B is a low-emissions diesel switcher locomotive built by MotivePower Industries. It is powered by three Cummins QSK19C I6 engines with each one developing 700 horsepower (522 kW) and creating a total power output of 2,100 horsepower (1,570 kW). One MP21B locomotive has been produced to date, currently operated by Amtrak.