This experimental streamline locomotive was designed for high speed. Long (15.36 m), heavy (127 ton), with a 3070-bhp machine and 2000mm wheels it could easily break the 100-mile barrier. Unfortunately, both Ks ("K" is for Kolomna, where the locomotives were built) had to stick to the low-speed schedule, rarely exceeding 70 km/h (44 mph). But if for some reason the train was behind the schedule, Ks were able to bring it to the destination right on time.
Their lifespan was short. Used in 1938-1940 for hauling the Red Arrow express between Moscow and Leningrad, both Ks ended their active service when the Soviet Union entered the war.