Having abandoned the idea of buying steam engines, given the slim length of the electric network, PKP started to introduce diesel traction. First, three diesel wagons (SD80) with hydraulic transmission and two motors, which could develop a speed up to 81 mph (130 km/h), were bought from OM of Milan, Italy. Fablok in Chrzanów released its first low-power Ls40 locomotives (44 HP) in 1952, based on the pre-war documentation. Fablok also built
300 HP locomotives with a working identification symbol Lwe55. In 1957 – 1970 the factory released 293 units. In addition, Polish State Railways were a regular customer of Ganz-Mavag – a Budapest-based manufacturer, from which over 20 years, starting from 1956, they bought 575 diesel wagons.
Diesel locomotives were also produced by the Cegielski Factory which in 1961 – 1964 released a series of 13 wagons of 5M350 type (SN80) with hydraulic transmission. The last one was taken out of service in 1979.
The basic type of shunting locomotive was the SM42. It was produced by Fablok in 1964 – 1992 for Polish State Railways and Polish industry (1822 locomotives). Its variation, 101D, equipped with a steam boiler fired with fuel oil for the heating of trains, was produced in 1971 – 1978 (268 vehicles). The SU42 locomotive with a heating generator was used for wagons fitted with electric heating only. The above-mentioned locomotives could pull lightweight freight and passenger trains.
Traction for passenger and freight trains required heavyweight line locomotives that were not produced in Poland. Therefore, starting from 1965, ST43 diesel locomotives manufactured under a Swiss licence were imported from Romania. They were mostly used in freight traffic service but in summer they also pulled passenger trains. Up until 1978, Polish State Railways bought 422 such locomotives.
Heavyweight freight locomotives were also imported from the USSR. Over 23 years, starting from 1965, a total of 1,181 locomotives were bought (including 68 broad-gauge locomotives for LHS). They were the most numerous series of freight diesel locomotives used by PKP, equipped with an uneconomical two-stroke diesel engine. The rolling stock was taken out of service and scrapped on a mass scale at the end of the 1980s. Currently, NEWAG installs modern Caterpillar engines on the chassis of those locomotives. Thus, following a change in equipment, electrical instru-ments and the body, the locomotives are converted into broad-gauge ones.
In the early 1970s diesel traction vehicles carried out 40% of PKP's shunting work and 17% of transport (steam loco-motives – 38%, and electric locomotives – 44%). However, there was a shortage of medium-duty locomotives for passenger train service, on secondary lines. Thus, the SP32 series was bought in 1985 – 1991 from Romania but it turned out to be very fallible. The requirement for heavyweight shunting loco-motives was satisfied by the production of the heaviest diesel locomotives used by PKP, i.e. type 411D, and TEM2 vehicles imported from the USSR part of which was used for broad-gauge shunting.
Since several dozen carriers have recently been allowed to provide their services on Polish railway lines, a great variety of locomotive models are in use.