In 1909, the PLM started comparisons with Pacfic locomotives with different types of power plants. The locomotive with the number 6001 had a saturated four-cylinder compound engine and a boiler pressure of 15 bars, while the locomotive initially given the number 6002 was built with a superheated single four-cylinder engine and a boiler pressure of only 12 bars. Although the compound engine had already emerged as the winner in the first comparisons, series orders of both versions were ordered for more precise comparisons under everyday conditions. While the 6002 was renumbered 6101, the first production examples of the 6001 were numbered 6011 to 6030.
In contrast to practice in Germany, the smaller high-pressure cylinders were arranged on the outside at the height of the second bogie axle. The low-pressure cylinders were inside and below the smoke box. Based on the De Glehn principle, both groups of cylinders each had their own valve gear. The high-pressure cylinders acted on the second coupled axle and the low-pressure cylinders on the first.
No. 6013 on a postcard
During the tests, the 6001 already showed that it could bring a 495-tonne train to its destination in just 87 minutes over a 160 km long route with an altitude difference of more than 300 meters. The 6002 had taken over ten minutes longer to cover the same distance with a slightly lighter train, using more coal. Nevertheless, it was found that the 6001 could have used larger cylinders.
Series production initially started in 1912 with locomotives 6011 to 6030, which were built by Henschel in Germany due to a lack of domestic production capacity. The diameter of the cylinders had been increased from 390 to 440 and from 620 to 650 mm and the cylinders were now all in a line at the height of the smoke box. They were factory built with a superheater and the 6001 was also fitted with a new superheated boiler in 1912.
In 1913 and 1914, 40 more examples came from the works on the Loire in Nantes and in 1921 another 25 from SACM. The first series was renumbered 6201 to 6220 and the later series locomotives were given the numbers 6221 to 6285. In the PLM's new numbering scheme from 1925, the prototype and series locomotives were collectively referred to as 231 C. In the SNCF they were then renamed 5-231 C with the appropriate regional prefix.