An auxiliary power unit and a method of providing auxiliary power to a locomotive are disclosed. The auxiliary power unit assembly for a locomotive includes an auxiliary power unit removably coupleable to a rail car chassis. The auxiliary power unit includes a housing, and an auxiliary engine-generator set positioned within the housing. The auxiliary engine-generator set is configured to provide an auxiliary power to the locomotive. The auxiliary power unit also includes an auxiliary controller electrically coupled to the auxiliary engine-generator set. The auxiliary controller is programmed to receive a command and control at least one aspect of the auxiliary power unit in response to the command.