This paper presents a study on the power conditioning features of hybrid powertrain, especially regarding diesel-electric locomotives. Equipped with an embarked energy buffer for diesel generator support and utility grid interface, such a locomotive can be considered as a plug-in series hybrid vehicle. The driveline inductive components, like generator and motor windings, and capacitive components like dc link capacitors can be used to provide STATCOM functions, and the energy buffer can be used for short-term grid support. The co-action of power electronics circuitries and diesel generator gives a possibility to observe the locomotive as a mobile versatile power plant for special rolling stock and emergency supply. With high power levels ranging from few hundred kilowatts to several megawatts, the locomotives can participate in large-scale power conditioning at standstill, bringing revenues to the operators even “off-line”.