This paper presents a hypothetical conversion of a conventional heavy haul diesel-electric locomotive to its hybrid counterpart by incorporating a battery energy storage system. Starting from the basic parameters of a 1.6 MW diesel-electric locomotive currently found in the national railway company locomotive fleet, the quasi-static model of the locomotive is derived and validated. The conventional locomotive model is then converted to its hybrid counterpart by adding a battery energy storage system in parallel to the generator and equipping it with an adequate optimized energy management control strategy. The hybrid locomotive powertrain components are also appropriately re-sized in order to meet comparable traction force and power performance. Both the conventional and hybridized locomotive models are then used for the purpose of comparative analysis of main vehicle characteristics for the mountainous railway route driving scenario, which includes realistic slope and speed limitations. The obtained simulation results are used to gain insights about the possible advantages of the proposed conversion/drivetrain hybridization in terms of feasible reduction of fuel consumption and related CO2 emissions, while also considering additional hybridization costs and return-of-investment period. (C) 2019 Elsevier Ltd. All rights reserved.