The 40-000s were of simple expansion with two cylinders and possessed a Crampton firebox and Walschaerts valve gear.

The water tanks on each side of the firebox possessed a sloping front and gave the driver additional vision and permitted him to see if workmen were on the track in front of the engine.

The war prevented the construction of the rest of the series, and the last two engines of the first batch, 40-019 and 40-020, were taken by the German Army and not returned until 1919 and 1922 respectively.

Construction resumed in 1920 and the full series of 143 engines was put into service by 1922.

The engines were assigned to the depots of Paris-Vaugirard, Montrouge, Batignoles, Sotteville (Rouen), Le Havre, Dieppe, Trappes, Chartres, Caen, Cherbourg, St-Brieuc, Brest, Nantes, Rennes and La Rochelle as well as industrial railways and harbours.