Purpose: To develop an approach to upgrading parts of a self-contained mainline freight locomotives fleet in order to reduce their life cycle cost and harmful emissions levels. Methods: The authors reviewed consumer and performance characteristics, investigated the potential for changes in the layout, drawings and design of mainline freight diesel locomotives projected for conversion to operate on a gas-diesel cycle. Based on the experience with operating gas-fueled locomotives, an introduction of cryogenic gas equipment has been proposed into the design of diesel locomotives due for conversion as well as a harmonization of the equipment types and its optimization. The upgrade criteria and conditions have been defined, and some solutions to minimize the number of changes to the conventional diesel locomotive design have been put forward. Results: The objectives of increased cost efficiency and improved environmental performance have been specified, the design solution proposed for the self-contained diesel locomotives fleet conversion has been justified, a study of gas-diesel locomotives’ energy efficiency and cost effectiveness has been performed, the criteria for selecting specific diesel locomotives for the upgrade have been proposed as well as the engineering changes to enable the conversion of diesel locomotives to operation on a gas-diesel cycle. Practical significance: The suggested approach to upgrading self-contained locomotives will reduce their life cycle cost, hydrocarbon emissions and minimize the duration and cost of conventional diesel locomotives conversion to operation on a gas-diesel cycle.