Diesel engines in railway locomotives are operated in various modes, depending on the function of the locomotive and its operating conditions. After analyzing scientific literature, the authors found that the type of locomotive has an impact on the engine's operating modes and on the engine's idle time. The literature emphasizes that switcher locomotives spend a large part of their working time idling. However, the authors of this study found that this problem is also relevant for mainline locomotives. This study examines the percentage distribution of operating modes` of the diesel engines of randomly selected ER20CF mainline locomotives used by Lithuanian Railways and assesses the possibility of applying “start-stop” systems in locomotives to reduce fuel consumption during idling. The study found that idling accounts for an average of over 50% of the total operating time of the locomotive engine. The study found that fuel consumption during idling corresponds to the consumption of an engine operating at about 14% of its power. After evaluating the idle time and fuel consumption of locomotives when the engine is idling, recommendations were made for the installation of a start-stop system.