Diesel locomotive
Diesel locomotives offer significant operating advantages over steam locomotives. They can safely be operated by one person, making them ideal for switching/shunting duties in yards (although for safety reasons many main-line diesel locomotives continue to have 2-man crews: an engineer and a conductor/switchman) and the operating environment is much more attractive, being quieter, fully weatherproof and without the dirt and heat that is an inevitable part of operating a steam locomotive. Diesel locomotives can be worked in multiple with a single crew controlling multiple locomotives in a single train — something not practical with steam locomotives. This brought greater efficiencies to the operator, as individual locomotives could be relatively low-powered for use as a single unit on light duties but marshaled together to provide the power needed on a heavy train. With steam traction a single very powerful and expensive locomotive was required for the heaviest trains or the operator resorted to double heading with multiple locomotives and crews, a method which was also expensive and brought with it its own operating difficulties.
Rolling stock of the Korean State Railway
Later, around the late 1960s or early 1970s, a system similar to that introduced for electric locomotives was applied to steam and diesel locomotives. Steam locomotives were universally given the type designator 증기 ("Chŭnggi", "steam") and diesel locomotives 내연 ("Naeyŏn", "internal combustion"), along with a three- (for mainline diesels) or four-digit (for shunting diesels and all steam engines) serial number, with diesel locomotives are numbered sequentially within their number series. On steam locomotives, the first of the new four-digit serial number reflects the former type designator (부러 → 1, 미가 → 6, 마더 → 7; the others are unknown, and the 8000 series is applied to steam locomotives of foreign origin supplied as war aid during the Korean War); the second digit reflects the former type-series, directly translating the number, e.g. 미가서 became 6300, 마더하 became 7100. The last two digits are the locomotive's original running number (last three digits, in the case of the Migaha class numbered into the 6000 and 6100 series). The system behind determining which numeral represents which type designator (that is, why Miga became "6", etc.) is unclear, as this numeral has no relation to anything obvious, e.g. number of powered axles, etc.