In order to obtain a more powerful express locomotive than those with the 4-6-0 wheel arrangement, the Reichseisenbahnen in Alsace-Lorraine had some of the first Pacifics built in Europe in 1908. However, in order to be able to use existing turntables and at the same time allow a large tender, the trailing axle had a very small distance from the third driving axle. 

This meant that the Belpaire firebox had to be moved between the driving wheels, so the G 12 was not one of the typical Pacifics with a wide firebox. Typical of French-influenced areas, they had four cylinders based on the de Glehn system. Other features included serve tubes in the boiler and an exhaust system with a variable cross-section and cloverleaf shape.

Since the boiler and firebox were too small compared to other Pacifics and ten-wheelers, only eight S 12 were built. The seven that remained after the First World War came to SNCF in 1938 as 1-231 A. After the Second World War, three came to Luxembourg, two to the GDR and two to Poland. While they were used in Luxembourg until 1950 and in the GDR until 1953, the two in Poland were decommissioned in 1946 without actually being used.