In the thirties, the Soviet State Railway wanted to increase the speed of express trains on the October Railway between Moscow and Leningrad. For first tests, a locomotive of the 2-8-4 class ИС (IS) was streamlined and designated IS20-16. Although this one reached 155 km/h, a pure express locomotive was to be designed with a driver diameter of 2,000 mm. So one driving axle was removed and a two-axle bogie was installed at the front, leading to the wheel arrangement 4-6-4 or 2-3-2 in Russian notation. This led to better running characteristics and they could be designed for a regular speed of 150 km/h.
The first prototype was completed in 1937, followed by the second one in 1938. They had roller bearings on all carrying axles and no combustion chamber to save weight. Both were streamlined, but only No. 2 had smoke deflectors. No. 1 had an experimental L40 superheater with large tubes, while No. 2 got an Elesco-E superheater with small tubes. Built at Kolomna, their internal designation was P12 and today they are mainly being known as 2-3-2K, but this was no official designation.
It did not take long until they reached 160 km/h, and soon even 170 km/h with only four cars. Indicated output of No. 1 at 150 km/h was 3,070 hp. Soon they were put into regular service alongside the Су (SU) 2-6-2 and could not show their full potential. Only when they had to make up for lost time, they could manage to cover 331 km in three hours. No. 1 was equipped with a trailing-truck booster in 1940, but no series production followed due to the war. After the war, the two prototypes lost their streamlining to save weight and were used at lower speeds.
