[1] The Class 685 was the simple-expansion and superheated version of the pre-existing FS Class 680 locomotive, the premier express locomotive in Italy up to that date; while discarding the complicated compound feature, the 685 nonetheless kept the unusual feature of having the cylinders arranged in pairs, with each pair being served by a single piston valve with crossed ports (a feature typical of the Plancher engine of the Class 680 and other Italian locomotives).

[4][2][5] The Class 685 was present on all the Italian mainlines before their electrification, thanks to its good performance related to its moderate axle load.

Despite being express locomotives, especially in the waning years of the Italian steam they proved to be apt for freight train pulling as well.

[10][11] The first group of 31 locomotives, renumbered as 685.3XX, received the superheater and a simple-expansion engine, while keeping the original boiler pressure of 16 kg/cm2 (15.7 bar; 228 psi); they resulted in being even more powerful than the original Class 685s, thanks to their higher boiler pressure.

[16][17][18] In 1933 the locomotive 685.410 had her cylinders replaced by a large forward motion steam turbine and a smaller reverse turbine; this experiment, under the guidance of renowned engineer Giuseppe Belluzzo, was completely unsuccessful, and the locomotive was restored to its previous condition in 1934.