The М was the heaviest passenger locomotive in the USSR when it was introduced in 1926. It had the wheel arrangement 4-8-0 and was one of the few Soviet three-cylinder locomotives. They had a high-pitched boiler and were constructed in a way that the tender carried some of the weight of the firebox. Some were fired with coal and some with oil.
Production first started at Putilov and later at Lugansk. They got the nickname “Marusya”. But soon it became evident that they didn't produce enough steam at speed and despite three cylinders, the running characteristics were not optimal. So in 1934, all were rebuilt to the two-cylinder MR by removing the inside cylinder and switching the cranked axle for a straight axle. Soon they were eclipsed by the much more powerful ИС and after World War II, they were only used for suburban service.