French Northern Railway No. 6.121 to 6.168

The Nord had to haul large quantities of coal from Lens in the Pas-de-Calais region directly to Paris, forming trains weighing more than 950 tons. These were pulled by locomotives with a 4-6-0 wheel arrangement, which achieved a high average speed on most of the route with a maximum incline of 0.6 percent. On the section between Hirson and Valenciennes, however, the gradient was 1.2 percent, where these locomotives were no longer sufficient.

A freight locomotive was now required that could transport the 950 tonnes on the steep section. Since the maximum axle load of less than 15 tons required many coupled axles, sufficiently large wheels were required for 80 km/h and curves with a radius of 90 meters or more still had to be negotiated, an articulated running gear was necessary. The solution to these requirements was developed by Gaston Du Bousquet and was based on the Meyer design.

The locomotive stood on two bogies, of which the rear one held the high-pressure cylinders and the front one the low-pressure cylinders. The cylinders were each on the inboard side to allow shorter steam paths between the cylinder groups. To circumvent the difficulties of moving steam pipes, the connecting tubes were made of rubberized canvas. To start off, the low-pressure cylinders could be supplied with live steam at a pressure of eight bars, which provided a very high starting tractive effort. In contrast to most articulated locomotives, the non-driving axles were on the inside of the bogies. 

In 1905, initially two prototypes were built, which reached speeds of up to 84 km/h on test drives. On a gradient of one percent they could reach 20 km/h with 1,000 tonnes, at 1.3 percent the same speed was still possible with 800 tonnes. In three batches, 46 more engines were built for the Nord. Since the good performance soon became famous, 13 identical engines for the Est and 38 for the Ceinture were built from 1910 onwards. Similar locomotives were built for the Beijing-Hankou Railway and for the Andalusian Railways.

Although the locomotives had been developed with a split chassis due to the tight curves, they soon had to fear competition from simpler tank locomotives with a 2-10-2 wheel arrangement. However, they were able to remain in service for a long time and were designated by the SNCF as 031+130 TA and TB. They were finally replaced by other engines by 1952, again by tank locomotives with an 2-10-2 wheel arrangement of the Est series T 151-751 to 780. In the meantime, there had been rebuilds on individual engines, including new superheaters, Kylchap blast pipes and new injectors.