When the French State Railways were already planning the complete conversion to electric and diesel locomotives, André Chapelon designed an express steam locomotive with an output of 4,000 hp at the drawbar. As a basis, he used the Est 241-101, which had not made it into series production, and modified it accordingly. He converted the simple three-cylinder engine into a compound engine and provided the locomotive with streamlined steam pipes and a Kylchap exhaust system.

The result was the most powerful steam locomotive ever built in Europe, producing between 5,230 and 5,430 hp (3,900 and 4,048 kW), depending on the source. While the original locomotive only had 2,550 hp on the drawbar, over 4,000 hp were now achieved and express trains heavier than 1,000 tonnes could be pulled. Up to 158 km/h were reached on test runs and approval was given for 130 km/h. The locomotive was put into service in 1951, but was only used sporadically due to the decision to electrify the network and was scrapped in 1961.