In 1929, André Chapelon used all the knowledge he had acquired up to this point to fundamentally rebuild Paris-Orléans Pacific No. 3556, built in 1912. The result was almost twice the power with the same coal consumption. Because of this success, the 3501 to 3520 series locomotives were also rebuilt in 1932. Ten more followed in 1934.
The measures included thicker steam pipes, thermic siphons, a new valve gear and an increase in superheating from around 300 to 410 degrees Celsius. In addition, an ACFI feedwater heater and a double Kylchap blast pipe were installed.
The indicated power achieved through the rebuild was up to 3,700 hp for a short time or 2,800 hp permanently on the draw bar. The maximum speed of 120 km/h (75 mph) permitted by the PO could be maintained permanently with an express train weighing more than 650 tonnes on gradients of 0.3 percent. Accordingly, test runs on a topographically appealing route resulted in an average speed of 115 km/h, since the speed never dropped significantly.
These successes also brought the Nord and the Est to order the same locomotives. A total of about 100 locomotives were created, which were either converted from old ones or built from scratch. These all came from the PO works in Tours and were therefore called “Reines de Tours” (Queens of Tours). When the SNCF was founded, the 21 PO locomotives that were built first were assigned to the 4-231 E series. The ten locomotives from 1934 became the 4-231 H. They were last used in 1967.