In order to accommodate a boiler with the largest possible heating surface between large drivers, one of the approaches was the Flaman boiler. This consisted of two pipes arranged one above the other, which were connected at several points. The lower pipe accommodated all heating tubes and the upper pipe served only as a water and steam room. The water level was kept roughly in the middle of the top tube, allowing relatively dry steam to be extracted without a large steam dome. This arrangement was first tested on a Crampton in 1891 and was a complete success because it achieved a great increase in performance.

Since the Crampton was no longer sufficient as an express locomotive at that time and the Est had therefore switched to the 4-4-0 wheel arrangement a few years earlier in this role, twelve locomotives with Flaman boilers were built in this wheel arrangement in the same year. In these, the lower tube of the boiler had a diameter of 1,200 mm and the upper one had a diameter of 800 mm. In order to keep the center of gravity low, the boiler was positioned low and the firebox was mounted low between the coupled axles. To make this possible, the wheelbase of the coupled axles was increased to three meters.

Since the first twelve locomotives proved their worth, a further 28 almost identical units were delivered between 1892 and 1894. A vacuum and an air brake were installed as braking systems at the same time. On a 443 km stretch eastbound from Paris, which had long gradients of 0.6 percent, the average speed with a 250 tonne train was 82 km/h. Since it was soon realized that a low center of gravity was not important even for express locomotives and that a boiler with a larger diameter could be placed higher instead, the Flaman boiler did not catch on.