Imperial Russian Railways series І

Shortly after the turn of the century, the Central Siberian Railway required powerful passenger locomotives for its winding mountain lines with light rails. In order to ensure sufficient mobility and to comply with the axle loads, the B^K was created in the Mallet design with a 2-4-4-0 wheel arrangement. Kolomna initially made five locomotives as compounds and four with four equal-sized cylinders, which had a slightly larger boiler with a lower pressure.

103 more followed until 1909, all of which were compounds. They were soon renamed to І by the Russian State Railways. They did not reach a larger number because they had a higher consumption and more complex maintenance than locomotives with a one-piece frame. Nevertheless, they remained in use for a long time, so that the last locomotive was not retired until 1954.