Class ALn 663 is quite similar, while maintaining the same mechanics, received a different classification exclusively for the new interior design that reduced the number from sixty-eight to sixty-three.

Much of FS's fleet was destroyed at the end of World War II, which meant the company needed a replacement diesel railcar.

The essential characteristic of the new rolling stock was the provision of the drive motor under the floor, so as to leave the maximum space available for passengers.

By building on the experience already acquired in 1954, the FS initiated the project for a new standard railcar, for which the FS established clearly the fundamental objectives: high reliability, ease of conduct and learning, maintenance and repair well-programmable, overall costs contained defined and relatively modest, given the kind of services that diesel railcars were called upon to play in a network where it had planned the electrification of all main lines.

Added to this advantage FIAT added the ability to use diesel engines derived from production in large series of trucks, a choice that proved decisive for the future success of ALn 668 resulting in a considerable saving in initial cost, ensuring maximum reliability and an efficient care service with high availability of spare parts.