PKP classification system (Polish locomotive designation) is a system of assigning letters and numbers to series and individual locomotives used by the PKP - Polish national railroad operator.

The system was introduced for the steam stock by the Ministry of Railways on 3 November 1922, shortly after Poland regained her independence (1918) when the Polish railroads inherited a variety of German, Austrian and Russian steam locomotives, each with its own type convention. It was put into use in 1923–1926 years. The adopted solution allows telling the locomotive type (passenger/freight/mixed), wheel arrangement, origin and some other information from the type designation. After World War II a similar system was also adapted for diesel and electric locomotives.

In the case of electric and diesel locomotives, and multiple units, a designation consists of two capital letters and two digits, without a space between letters and digits. The first letter E or S indicates a kind of traction (electric or diesel), the second letter indicates a locomotive purpose. Digits mark subsequent classes, and their ranges also carry additional information on construction features. Some railcar classes have three digits.

In the case of electric and diesel locomotives consisting of two cars, the letters A and B were added after the serial number for each car but the number is still the same for both the cars or after important modernisation (look for EU07A).

Every car in a multiple unit is further designated by its own suffix:

If there is more than one car of a given designation in a unit, they are further identified with the letter 'a' or 'b'. For example, a typical 3-car EMU class EN57 consists of the following cars:

Another 3-car EMU, class EW58, consists of the following cars:

Example of a single car's designation: EN57-830ra.