2-car diesel multiple unit ALn 668 series 3100 in original light blue/beige livery, curved front windows

In the second half of the 1950s, FS developed the project for a new "standard" diesel multiple unit with the aim of replacing steam traction and the older multiple units that had already proven themselves during the war on ordinary local routes, which, although unremarkable, were fundamentally important for the entire national railway traffic.

The simplicity of the design, reliability, and significant improvements in performance and comfort compared to previous vehicles made the new ALn 668 (68-seat suburban trains with multiple control) a great success, resulting in 785 units being built for FS alone over nearly three decades in 12 series, along with about 90 units for private railways, their trailers, and hundreds of units built on their basis for foreign railways. Starting from the series 1900 and 1000 from 1975/76, the body was "redesigned" by enlarging it and adding two paracentral vestibules.

The introduction of two paracentral vestibules and then the return to a single central double vestibule, similar to that of the first series, for the subsequent series 1200 and 3000/3100/3300 starting from 1979.