As a replacement for four-coupled express locomotives, the Est procured a total of 178 locomotives with a 4-6-0 wheel arrangement from 1906 onwards. This wheel arrangement was chosen because it had already proven itself in the passenger locomotives built between 1897 and 1902 with numbers 3401 to 3500. The diameter of the coupled wheels was increased from 1,750 to 2,090 mm and the four-cylinder compound engine was retained. The bogie, which could be moved sideways by 53 mm, was also equipped with brakes. After the 3101 and 3102 prototypes, the production locomotives were made more powerful, moving them from the Est performance class 9 to 11. Production took place in eight batches at Épernay, Maffei and SACM between 1906 and 1913 and a ninth batch at Batignolles-Châtillon between 1925 and 1927.

The locomotives of the first series did not initially have superheaters, but were retrofitted with superheaters from different manufacturers between 1912 and 1914. Later locomotives already had superheaders ex works. The superheaters were subsequently unified to simplify maintenance. Various approaches have been tried to increase the power or reduce consumption of individual locomotives. An additional steam dryer was installed between the high and low pressure cylinders on seven locomotives and one received a boiler pressure of 20 bars, but was later operated again at 16 bars due to the more difficult maintenance. Others received differently shaped smokebox doors or paneling over the steam domes. 

No. 3175 on an old post card

Pol Gillet

A comprehensive modernization took place between 1932 and 1946, which, in addition to the revision of the steam lines, also included a boiler pressure of 18 bars and other improvements. From 1938 the 28 locomotives that were not rebuilt became the 1-230 J and the rebuilt ones became the 1-230 K. While the first decommissioning began in 1950, twelve units were further modernized, converted to oil firing and provided with a streamlined fairing for the special rubber-tyred Drapeaux express trains. In this role they were later replaced by Pacific locomotives.