The China Railways JS () are a type of "Mikado" type steam locomotives, they were manufactured for use on mainline freight trains, passenger trains, as well as for heavy shunting.
The JS class was first developed in 1957 at the Dalian Locomotive Works, as freight locomotives. The JS design was an evolution of the JF1 class, and it retained multiple JF1 features, including the chassis. But it also incorporated features from the QJ class 2-10-2s: a Soviet-inspired all-welded boiler, Trofimov valves, a Worthington feedwater heater above the smokebox, oil lubricators for the axleboxes, and an eight-wheeled welded tender design.
The JS was also more powerful than the JF1s, with a boiler pressure of , a superheater area of , a tractive force of , and a driving wheel diameter of . The JS designation () possibly got its influence from a slogan that lead to the Great Leap Forward ().
Dalian constructed the first prototype JS in July 1957, and production on the class began in September that same year. From 1958 to 1961,ÃÂ Qishuyan Locomotive and Rolling Stock Works and the Beijing 7th Feb Works collectively produced 278 JS locomotives. Dalian ended their production in 1961, having built 757. From July 1963 to May 1965, the Datong Locomotive Works built 100 JS locomotives, and they brought the original JS total to 1,135. The visual appearances of the JS classes varied: they had varying lengths for their front steps; most would be built with a skyline casing that covered the chimney; and some were fitted with varying sizes of smoke deflectors.
The JS locomotives were initially used for mainline freight services, but in later years, they were mostly reassigned to secondary passenger services, short-distance transfer services, yard shunting, and services in industrial areas. In the latter, the JSs would commonly shunt and haul trains slightly too heavy for SY 2-8-2s.
In 1981, Datong, which by then was one of only two remaining Chinese factories to mass-produce steam locomotives, resumed JS production. At that time, some minor alterations were made to the JS design, including the removal of the mechanical stoker from JS-6240-onwards, and 358 locomotives were built accordingly. In 1986, the JS design was revised with major alterations, and they received a 'B' specification for them. The alterations included their alligator crosshead guides being replaced with single slide bar guides; their boilers being smaller in diameter; their feedwater heaters being removed; their tenders being modified with a greater capacity and simpler bogies; and the locomotive weight overall being made lighter than the older JSs by . 423 locomotives of the revised design were built until 1988. The final total of JS locomotives of all types produced was 1,916.
Mainline service for the JS classes ended in late December 2005 when the railway was dieselized, however, several of them would continue in service for some industrial firms, such as the Sandaoling Coal Mine Railway in the Xinjiang Uyghur Autonomous Region.
On 25 April 2022, when Sandaoling discontinued their regular steam operations, they continued to use their JS classes as backups to their Diesel motive power. The last JS class, JS-8089 that operated at Sandaoling, was retired from service on 15 January 2024, which officially ended all regular steam operations in China.
Locomotives produced from 1957 to 1965 were numbered JS-5001 to JS-6135. Starting at 5001 avoided overlapping the numbers of the various types of JF occupying the 1 - 4100 (approx) range. Locomotives produced at Datong in the early 1980s were numbered JS-6201 to JS-6558; after the introduction of the revised 'B' specification the locomotives were numbered JS-8001 to JS-8423. A small number of locomotives operating outside the scope of the ministry of railways (industrial railways) received different number designations.