This report, comprised of a literature review and experimental investigation, summarizes the current use, feasibility, and GHG-reduction potential of dual fuel hydrogen-diesel combustion for locomotive applications. The literature review investigates the current use of hydrogen-diesel combustion in rail and other modes, outlines the mechanics and performance of dual-fuel combustion, and assesses the technology's maintenance and retrofit effort. The experimental investigation tests the combustion and emissions performance of a heavy-duty compression ignition diesel engine, modified to include port-injected hydrogen, under locomotive-like engine loading conditions.