In 1925, the Est was the first railway in Europe to get a 4-8-2 “Mountain” locomotive. This prototype, numbered 41-001, was built in the Épernay workshops to haul heavier express trains than the Pacifics, since more and more cars were built from steel instead of wood. The requirements said that it should haul 700 tonnes at 115 km/h. Designed by Émile Duchâtel, it was a four-cylinder compound with a deep and narrow Belpaire firebox. First tests showed that it produced less than 2,000 hp. 

After trials with the Nord Pacific 3.1214, the prototype was rebuilt in 1928. The diameter of the low pressure cylinders was increased from 610 to 660 mm, the diameter of steam lines was increased, the boiler pressure was increased from 16 to 17 bars and the connecting rods were reinforced. Finally, the blast pipe was replaced with a variable cloverleaf variant. Now, the output of the locomotive was 2,450 hp. Later it was renumbered to 241-001.

Series production started in 1930 after the Est had ordered 49, with a further order for 40 by the État. Production was completed in 1935 and the last batches already had modern features like ACFI feed water heaters or Kylchap exhausts. Designed for 120 km/h, their top speed had to be reduced to 90 km/h after a derailment in 1932. After investigations, it was again raised to 105 and finally 110 km/h.

While the État was satisfied with their power, the Est started to rebuild them after the principles of André Chapelon as early as 1933. They raised the boiler pressure to 20 bars, decreased the diameter of the high pressure cylinders from 450 to 425 mm, applied internal streamlining and added blast pipes of the six-jet cloverleaf design to most locomotives. Only one received a double chimney. An ACFI feed water heater was installed in most locomotives, but three got one of the Worthington type. Now they developed an output of up to 3,500 hp, but the SNCF soon reduced their boiler pressure to 18 bars.

The SNCF designated the Est locomotives 1-241 A, while the ones from the État became 3-241 A. They had more traction than Pacific locomotives and especially the rebuilt ones were more powerful, but they were slower. So their tasks were very heavy express trains or trains on hilly lines. The first ones were withdrawn in 1952, but the last ones only in 1965. Today 241 A 65, the former État 241-001, is operational and stationed in Switzerland. The prototype 241 A 1 is on display in the Cité du Train in Mulhouse.