Nowadays,the interest in hybrid vehicles is constantly increasing,not only in the automotive sector,but also in other transportation systems,to reduce pollution and emissions and to improve the overall efficiency of the vehicles.Although railway vehicles are typically the most eco-friendly transportation system,since commonly their primary energy source is electricity,they can still gain benefits from hybrid technologies,as many lines world-wide are not electrified.In fact,hybrid solutions allow ICE-powered (internal combustion engine) railway vehicles,such as diesel multiple units (DMUs),to operate in full-electric mode even when the track lacks electrification.The possibility to switch to full electric mode is of paramount importance when the vehicle runs on urban or underground track sections,where low or zero emission levels are required.We conduct the feasibility study of hybridization of an existing DMU vehicle,designed by Blue Engineering S.r.l.,running on the Aosta-Torino Italian railway line,which includes a non-electrified urban track section and an electrified underground section.The hybridization is obtained by replacing one of the diesel generators installed on the original vehicle with a battery pack,which ensures the vehicle to operate in full-electric mode to complete its mission profile.The hybridization is also exploited to implement a regenerative braking strategy,which allows an increase in the energetical efficiency of the vehicle up to 18％.This work shows the sizing of the battery pack based on dynamic simulations performed on the Turin under-ground track section,and the results demonstrate the fea-sibility of the hybridization process.