The designation OKl27 stands for a passenger (O) tank locomotive (K) with the wheel arrangement 2-6-2 (l), which was designed in 1927. They were the first locomotives designed and built entirely in Poland and a total of 122 were built between 1928 and 1933. In terms of appearance, they were reminiscent of typical German steam locomotives, just when you look at the driver's cab with the bent side walls. They were delivered by H. Ceglielski from Posen.

Thanks to its symmetrical axle arrangement, it was equally suitable for fast driving in both directions, which made it suitable for the use on suburban railways. When it was introduced, it was the heaviest 2-6-2 tank locomotive in Europe and the maximum axle load was around 18 tonnes.

With good coal, the locomotives were able to pull a passenger train weighing 350 tonnes at 80 km/h on level ground, on a gradient of 1.4 percent this was still 60 km/h with 130 tonnes. Although the running characteristics became a bit unsteady from 50 km/h, this hardly disturbed the operation on suburban trains with many stops. After the invasion from the Germans, 120 of the locomotives were operated by the Reichsbahn as class 75¹². After the end of the war still 95 engines remained in Poland, which were used by the PKP in the areas around Warsaw and Katowice. After retirement from passenger train service, some were sold to private railways. Today four pieces are preserved, half of which are set up as monuments and half in museums.