Conventional APUs for diesel-electric locomotives may include an AC electric generator and typically require additional hardware to be installed to convert the AC power output by the generator to DC power that can power electrical systems or charge batteries in the locomotive. According to some embodiments, there is provided an auxiliary power unit (APU) or system for operation in cooperation with a primary engine. The APU includes a secondary engine; a primary engine coolant heating system, or a primary engine lubricant heating system; a control system that automatically shuts down the primary engine and starts the secondary engine responsive to a predetermined condition; and a Direct Current (DC) power generator that generates an output voltage, the DC power generator being driven by the secondary engine.