A methodology is given for assessing the energy efficiency of using locomotives under operating conditions based on data from decoding speed tapes and driver route sheets, taking into account additional recommendations that clarify the calculations of energy (thermal technical) indicators of the transportation work of locomotives along the route of rolling stock. Analytical expressions are proposed to substantiate the parameters of the specified recommendations by  the speed measuring tape - the values of the mechanical work of the braking forces and the forces of the main resistance to the movement of the train, necessary to identify the force  load of locomotives, as well as the actual coefficient efficiency of the technological process of transportation work of locomotives and an algorithm for identifying locomotives with inefficient operating power plants during the implementation of railway transportation of goods and passengers. A preliminary analysis of the research results shows that the criterion for determining the timing of maintenance and current repairs of diesel locomotives based on their linear mileage is not sufficiently justified.