The PLM operated some main lines on which important express trains with high passenger numbers had to be hauled. One of these was the north-south connection from Calais to the Riviera, on which the “Train Bleu” ran with many passengers from Great Britain. Since the Pacifics were not sufficient there, one of the first Mountain locomotives in Europe was developed in 1925 with the 241 A 1. The requirements included that they should be able to haul 600-tonne express trains at 110 km/h on the level and at 75 to 80 km/h on the line between Laroche and Dijon with gradients of 0.4 to 0.8 percent.
As a compound locomotive based on the De Glehn design, the low-pressure cylinders were on the outside outside of the bogie, while the inside high-pressure cylinders were moved further to the rear. They drove the first and second driving axles, which led to very short connecting rods. The bogie had a play of 61 mm to each side, the second driving axle had wheel flanges reduced by 21 mm, the third driving axle could be shifted 21 mm to each side and the trailing axle was an Adams axle which was radially adjustable by 96 mm to each side. The smoke box door was cone-shaped for better streamlining.
After the prototype there were the production locomotives No. 2 to 80 also from Schneider, the 81 to 95 from Marine et d'Homécourt, the 96 to 115 again from Schneider and finally two more deliveries from Marine et d'Homécourt, with the numbers 116 to 125 and 126 to 145. The boiler dimensions of the production locomotives had been changed slightly, the superheater had even been reduced from 105 to 86 m². A boiler output of 2,180 hp was calculated for a run with a 625-tonne express train on the line between Laroche and Blaisy-Blas. Even with an 850-tonne express train, it was still possible to reach 70 km/h on an incline of 0.5 percent.
From 1932 a total of 48 of the 145 locomotives were rebuilt to achieve higher efficiency and greater power. The diameter of the connecting pipes between the high and low pressure cylinders was increased and a double cloverleaf exhaust system was installed. In addition, large smoke deflectors were fitted and the cone-shaped smokebox door replaced with a flat one. The locomotives rebuilt in this way were classified as 241 D, with the running numbers being retained. At the SNCF, the two classes became the 5-241 A and 5-241 D.
