The integration of more environmentally benign fuel options for mass transportation applications is a critical research initiative, particularly those that support and further development toward a hydrogen economy, a key target for future sustainability. The thermodynamic and environmental performances of a hydrogen assisted hybrid locomotive with ammonia-diesel co-fueling are comprehensively assessed through energy and exergy analyses, and impact assessment. Onboard hydrogen production via thermal ammonia decomposition (cracking) utilizes exhaust gases as a heat source, reducing the total diesel fuel consumption and increasing energy utilization of the fuel burned in the locomotive prime mover engine. The hybrid system performance is compared against a conventional diesel-electric locomotive through energy and exergy efficiencies, fuel consumption, and environmental impact to evaluate the viability of the hybrid locomotive as a clean rail transportation option. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.