This series, numbers 내연601 to 내연665 is made up of the 64 units delivered new to North Korea from the factory, as well as one unit obtained second-hand from the Polish State Railways (내연665).
내연656 (works number 1384, built 1971), although built for the Korean State Railway, wasn't delivered immediately, instead having served for some years with Soviet Railways, assigned to the Lvov region as M62-1265.
Some of the units originally built for the DPRK have since been retrofitted with exhaust silencers.

Some units of this series are being modernised, which includes the replacement of the original Kolomna 14D4 engine.
The modernisation package includes replacement of the original engine with the new Kolomna 12ChN26/26 engine, and of the original 4-14DG generator with the new 5-26DG.
These have a much lower fuel consumption than the originals, and can go longer before overhaul is required.
In 2000, 내연604 was sent back to Luhanskteplovozstroi to undergo this modernisation, at which time it received a new blue and white livery similar to that used by the Russian Railways.
Additionally, it received new headlights like those found on the Russian DM62 and M62U variants.

Since 2006, the new 12ChN26/26 engines and 5-26DG generators have been delivered to North Korea, the modernisations now taking place in domestic shops.
So far, 내연602 - the machine inspected by Kim Il-sung and which carries plaques commemorating the occasion - is known to have been modernised.
Locomotive 602 also has the distinction of having been the first North Korean locomotive to cross the Korean Demilitarized Zone since the end of the Korean War.

Almost all locomotives of the 600 series are painted in the standard blue over dark green livery, although there are a few exceptions.
604, as mentioned, was repainted into a blue and white scheme used by the Russian Railways.
Several, including 622, 639 and 658 are painted in a scheme becoming more common, which replaces the blue in the standard livery with light green.
629 is overall dark green with a white stripe, and, as mentioned above, 656 still carries its Soviet paint and even lettering, while 653 has been repainted into the green and yellow scheme used on the 700 series locomotives.
The one locomotive in this series that wasn't ordered new, 665, is one of the very few second-hand units that received the standard blue over green livery.

Known to be assigned to P'yŏngyang Depot are 602, 619, 620, 622, 625, 627, 638, 650, and 664.

One locomotive was rebuilt, likely after an accident, with new cabs similar to those on the Red Flag 6-class electrics locomotive, but with dual headlamps; this is numbered 내연001 and is painted in the standard livery.
This series, numbers 내연601 to 내연665 is made up of the 64 units delivered new to North Korea from the factory, as well as one unit obtained second-hand from the Polish State Railways (내연665). 내연656 (works number 1384, built 1971), although built for the Korean State Railway, wasn't delivered immediately, instead having served for some years with Soviet Railways, assigned to the Lvov region as M62-1265. Some of the units originally built for the DPRK have since been retrofitted with exhaust silencers.
Some units of this series are being modernised, which includes the replacement of the original Kolomna 14D4 engine. The modernisation package includes replacement of the original engine with the new Kolomna 12ChN26/26 engine, and of the original 4-14DG generator with the new 5-26DG. These have a much lower fuel consumption than the originals, and can go longer before overhaul is required. In 2000, 내연604 was sent back to Luhanskteplovozstroi to undergo this modernisation, at which time it received a new blue and white livery similar to that used by the Russian Railways. Additionally, it received new headlights like those found on the Russian DM62 and M62U variants.
Since 2006, the new 12ChN26/26 engines and 5-26DG generators have been delivered to North Korea, the modernisations now taking place in domestic shops. So far, 내연602 - the machine inspected by Kim Il-sung and which carries plaques commemorating the occasion - is known to have been modernised. Locomotive 602 also has the distinction of having been the first North Korean locomotive to cross the Korean Demilitarized Zone since the end of the Korean War.
Almost all locomotives of the 600 series are painted in the standard blue over dark green livery, although there are a few exceptions. 604, as mentioned, was repainted into a blue and white scheme used by the Russian Railways. Several, including 622, 639 and 658 are painted in a scheme becoming more common, which replaces the blue in the standard livery with light green. 629 is overall dark green with a white stripe, and, as mentioned above, 656 still carries its Soviet paint and even lettering, while 653 has been repainted into the green and yellow scheme used on the 700 series locomotives. The one locomotive in this series that wasn't ordered new, 665, is one of the very few second-hand units that received the standard blue over green livery.
Known to be assigned to P'yŏngyang Depot are 602, 619, 620, 622, 625, 627, 638, 650, and 664.
One locomotive was rebuilt, likely after an accident, with new cabs similar to those on the Red Flag 6-class electrics locomotive, but with dual headlamps; this is numbered 내연001 and is painted in the standard livery.
K62-class locomotive