Dieselisation
Weighing against the cost of, and inertia against, replacing the large investment that railroads had in existing steam power were the dramatic increases in flexibility and efficiency with diesel. The fastest and most powerful steam locomotives were faster and more powerful than diesels; however, their range of efficient operation was severely limited. Diesels pro-rate their fuel usage to the length of trains, which a steam engine cannot do. Multiple-unit diesel power is scalable to power requirements with one locomotive crew; steam power is not. A high speed Hudson steam locomotive is good for only one situation, high speeds on level grades. The diesel locomotive can be operated by a single person, with no need of a fireman to run the boiler (two person cab crews may be required for other reasons). Also, diesels use much less fuel and no manpower when idling, something locomotives often do. Diesels can be parked running for days unattended, whereas steam engines must be constantly tended to if not completely shut down. Bringing a steam engine boiler up to operating temperature is often regarded as both an art and science, requiring much training and experience. A diesel is much simpler to start and shut down.
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.