The letter Э (E) designated a family of tender locomotives with a 0-10-0 wheel arrangement on the Russian and Soviet railways. With a number of more than 10,000 and a production period of 45 years, it is the most-built steam locomotive of all time. Not only factories in Russia and the Soviet Union were involved, but also factories in other European countries.

The design came from Wacław Łopuszyński from the Vladikavkaz Railway. Here, for the first time, five coupled axles were necessary because the axle loads were limited and a sufficiently strong freight locomotive was required for the increasing train weights. The Gölsdorf system was used here, in which some of the axles could be moved sideways. Other requirements included a Belpaire firebox and a Schmidt-type superheater. 

The first 15 locomotives were delivered from Lugansk in 1912 only to the Vladikavkaz railway. Other railway administrations soon followed and from 1915 production was also handed over to other works. The cylinder diameter increased from initially 600 to 630 to 650 mm. A total of 1,528 of the basic variant had been manufactured by 1925.

Since industrial capacity had declined significantly after the October Revolution and a large number of freight locomotives were needed for the reconstruction, manufacturers were now sought in the West. They first turned to NoHAB in Sweden, where 1,000 copies of the Э^ш were to be manufactured. The letter ш (Sch) stood for “Sweden”. The first 18 locomotives were delivered by rail in a cumbersome and time-consuming process, since the track gauges differed from country to country. The rest was then transported by ship.

Although many subcontractors were involved in the production, the ordered 1,000 locomotives could not be delivered. Now a large number of German manufacturers were also taken on board, so that ultimately between 1921 and 1924 500 were manufactured in Sweden and 700 in Germany. The German locomotives were designated Э^г, with the г (G) standing for “Germany”.

Production capacities were soon available again in the Soviet Union, but the industry was not yet ready for a complete redesign. Thus the Э^У (E^U) was developed, which achieved an additional performance of 15 percent thanks to a newly developed superheater. 2,535 of these were made between 1926 and 1931.

This was followed by the Э^М (E^M), which was primarily adapted to the increased transport volumes with a higher boiler pressure. Since weight had been saved in some places by welding and the omission of the feed dome, the axle loads were distributed unevenly. Nevertheless, a total of 2,325 of this variant were made between 1931 and 1935.

The problems with the Э^М were solved with the Э^Р, which had a longer firebox and again a feed dome. In addition, it achieved an increase in output of ten to 15 percent with an eight percent increase in efficiency. During World War II some came to Finland where they were used as Tr2. Since the Soviet industry was no longer able to produce sufficient quantities of freight locomotives itself after the end of the war and no new design was possible, further Э^Р were built in their own country and also in other countries of the Warsaw pact. When production ended in 1957, a total of 3,044 Э^Р had been built.

Due to its large number, the Э was distributed throughout the country. It was not only used in front of freight trains, but also in front of passenger trains. Over the years it has been used for various trials, such as thermic syphons, improvements to the firebox or pulverized coal firing. Although production continued into the 1950s, many of the older variants were already being moved to shunting service or handed over to industry during this period. In the sixties the decommissioning started in larger numbers.