The first family of new SNCF wagons, DEV AO (Ordinary Steel) wagons, brought a new look to passenger coaches, with access doors that are in the plane of the side walls, encircling aprons that even covered the bellows, retractable steps and a circulating air heater for greater passenger comfort. The speed record of 331 km / h was set in March 1955 with a locomotive-hauled train consisting of 3 DEV AO cars. The DEV AO wagons, built in 1605 copies, were used for long-distance trains from 1950, from which they were later replaced by the UIC Y-wagons and then Corail wagons.

DEV AO cars, all with compartments, come in two series: "short" DEV AO cars and "long" DEV AO cars, modeled by Arnold.

The development of the long DEV AO wagons was an indirect result of the changeover to two classes in June 1956. Among the short DEV AO wagons already built, the C10 wagons became B10s, while the B9c9 couchette wagons remained B9c9 wagons.

In order to maintain the same capacity with 10 day compartments and compartments that can be converted into berths with the same comfort conditions, a new series of extended DEV AO passenger coaches (B10c10, then B10, A9 and A4c4B5c5) has been introduced.