A locomotive assembly including a legacy locomotive controller and an intercept locomotive controller and a method of controlling a locomotive are disclosed. The locomotive assembly includes a locomotive having a power bus, a primary power unit coupled to the power bus, and a legacy locomotive controller programmed to transmit control signals to the primary power unit. The locomotive assembly further includes an intercept locomotive controller programmed to receive a control signal indicating an requested amount of locomotive power from the legacy locomotive controller, allocate a portion of the requested amount of locomotive power to an auxiliary power unit, and control the auxiliary power unit to deliver the portion of the amount of locomotive power to the power bus.