N° M 31 is a 4-wheeler, single-cab, unidirectional railcar, built in 1954 by the VFIL works at Lumbres (CGL company) and was originally assigned to the “Flanders network” in the Nord department. This network closed in 1957. M 31 was then bought by the Somme department, to complement the existing railcar roster, more recent than the pre-war units. It carried out the same type of service as the other railcars, except that it had to be turned systematically on the turntables located at the end of each line. It was mainly assigned to Noyelles-Le Crotoy, where it worked the very last commercial train in the evening of 31st December 1969. Having been created in 1970 to take over operation of this line, the CFBS preservation society bought M31 in an early, but unsuccessful, attempt to provide summer tourist services. This was one of the very first railway vehicles to be preserved by the CFBS. Affectionately nick-named "Blup-blup", the railcar was put into service in 1971 and provided the CFBS with sterling services. In the early 1980s, however, it had to be withdrawn due to an engine breakdown and excessive wear of its wheels. Much work will be required to bring this piece of technical heritage back to life, sometime in the future…

Some technical data:
- Weight empty: 11 metric tons.
- Wheel diameter: 0.6 m.
- Length:9.4 m.
- Width: 2.4 m.
- Height: 3 m.
- Motor: 4 cylinder 100hp Willème diesel
- Transmission: mechanical.
- Maximum speed in service: 40 Km/h.