In 1940 and 1941 André Chapelon rebuilt 25 Pacifics of the PO 4501 to 4600 series into 4-8-0 locomotives of considerably more power which became SNCF 240 P. They were a follow-up of twelve other locomotives of the same series which had been rebuilt in 1932 and 1933 which now became SNCF 240 A. The new series was to be even more powerful and used for very heavy passenger and freight services on the line between Paris and Lyon with gradients of one percent.

Innovations of these locomotives included a boiler pressure of 20 bars, a stoker, an ACFI feed water heater, a longer stroke on the low pressure cylinders and a double Kylchap exhaust. This resulted in a maximum output of around 4,700 hp and a continuous output of around 3,800 hp. On the outside, the 240 P could be distinguished from the 240 A by running boards above the driving wheels, larger smoke deflectors and a four-axle bogie tender.

Heavy passenger trains of 800 tonnes could be hauled at speeds above 100 km/h, what increased to 120 km/h with trains of 512 tonnes. In World War II, passenger trains grew to 28 cars or 1,100 tonnes, while these locomotives were also tasked to haul 2,000-tonne coal trains at 85 km/h. But the electrification of the Paris-Lyon line meant that their lives were fairly short, so the last ones were withdrawn in 1953.