Diesel fuel remains the dominant energy source for heavy railway traction worldwide, yet its cost constitutes up to 35 percent of locomotive operating expenditure and contributes substantially to NOx, particulate matter, SOx, and CO2 emissions. Gas-diesel dual-fuel technology, which simultaneously supplies compressed or liquefied natural gas together with a diesel pilot charge, offers a documented path toward cost reduction and cleaner combustion; however, its application to mainline locomotives presents open engineering questions that are insufficiently studied relative to marine or stationary counterparts. This article examines how natural gas fuel quality , expressed through methane number, Wobbe index, and sulfur content , affects in-cylinder combustion, brake thermal efficiency, reliability of critical engine components, and regulated pollutant output in locomotive gas-diesel engines. Using comparative analysis of peer-reviewed experimental data, technical case studies from commercial deployments, and author-proposed adaptive control logic, the study demonstrates that a gas substitution ratio of 60–70 percent at cruise load reduces fuel expenditure by approximately 40 percent while cutting NOx by up to 67 percent, SOx by up to 100 percent, and particulate matter by up to 90 percent relative to diesel-only operation. The findings are relevant to locomotive design engineers, railway operators pursuing decarbonization, and regulatory authorities setting emission standards for non-road mobile machinery.

: Gas-Diesel Dual-Fuel Engine, Locomotive, Methane Number, Wobbe Index, Gas Substitution Ratio, Nox Emissions, Brake Thermal Efficiency, Heat Release Rate, Adaptive Fuel Management, Railway Decarbonization.

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