A few years after the revolution, China Railway developed a Mikado that would be built in large numbers. The chassis of the JF1 class, which had been built since 1918, was used as the basis. It received a modern boiler and was also adapted to current requirements in other areas. The new class was called JS or “Jiàn Shè”, which roughly means “construction”.

With a coupling wheel diameter of only 1,370 mm, a high level of traction could be achieved. As is usual for China, the tender was chosen to be large with a coal supply of 17 tonnes. The design came from Dalian, but the production was subcontracted to several plants.

The first 1,135 units were built between 1957 and 1965. Another production only started in 1981, which included almost 800 more locomotives until 1988. More than half of them belonged to a “B” variant, which received some improvements. A special feature was a locomotive that was sold new in 1989 to the Boone and Scenic Valley Railroad in Iowa.