As the electrification of the Korean State Railway's network continued into the 1980s, the Red Flag 1 and Red Flag 2 class locomotives were the dominant type in service under the wires. However, with the electrification of steep mountain lines, it was realised that a more powerful locomotive was needed. Therefore, the Kim Chong-tae Electric Locomotive Works began design work on an articulated locomotive to operate freight trains on the difficult mountainous sections.[3]
Using the Red Flag 2 class as a starting point, the Red Flag 6 (붉은기6, Pulg'ŭn'gi-6) class was developed.[3] The result was a permanently coupled Bo'Bo'+Bo'Bo' articulated locomotive with eight capacitors,[4] made up of two sections; these sections are single-cab versions of the Red Flag 2. The Red Flag 6 class is thus an articulated descendant of the Škoda Type 30E, which had formed the basis for the design of the Red Flag 1 and Red Flag 2 after North Korea bought a licence to build the type, including technology transfer, from Czechoslovakia.[3] The first prototype was shown in 1981.[5]
Numbered in the 6000 series, production of the new design began in 1986, and was put into service in 1987 to haul freight trains on the P'yŏngra Line.[6] These four-bogie, eight-axle articulated units are 33 metres (108 ft 3 in) long, weigh 176 tonnes (388,000 lb) and produce 4,240 kilowatts (5,690 hp). They have a maximum speed of 100 kilometres per hour (62 mph) and can pull 3,200 tonnes (7,100,000 lb).[7] Trial runs with passenger trains were undertaken, but the type is now mainly used on east-west freight trains on the P'yŏngra Line.[3] The first prototype received a cream over red paint scheme, but production units are painted in the standard light blue over dark green livery.
