Shunting is an important component of railway transport, a significant part of which is performed by diesel locomotives. Piston engines are the most common locomotive power machines. The paper provides brief information on the trends in modernization of energy machines for diesel locomotives. It also considers the issues of improving the working processes of engines converted to gas fuel on the basis of numerical modeling. The main goal of the simulation is to improve the performance of gas reciprocating engines. The authors propose the option for refinement of the working process for three gas reciprocating engines that decreased the specific fuel consumption to 11.5 % and NOx emissions by 2-10 times while maintaining power within ±10 % compared to basic diesel engines.