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. The first 106 locomotives were built up to World War I, and had right-hand drive; construction resumed in 1920, and the remaining 165 were all built, up to 1928, with left-hand drive and a slightly longer boiler. Of the latter, 30 were built from the beginning with Caprotti valve gear, after it had been tested first on a FS Class 740 locomotive and then on other four 685 locomotives; all of these were initially reclassified as FS Class 686 locomotives, but were later placed again into the Class 685.
Operations
The Class 685 was present on all the Italian mainlines before their electrification, thanks to its good performance related to its moderate axle load. It was renowned as a well-balanced design, especially for its very smooth riding qualities, and was almost universally appreciated. Despite being express locomotives, especially in the waning years of the Italian steam they proved to be apt for freight train pulling as well. The maximum authorised speed for the Class 685 locomotives was originally and from 1932, it was raised to. The first units to be withdrawn from service, in the mid 1960s, were the heavier S.685 locomotives, and the more maintenance-requiring Caprotti and Franco-Crosti locomotives; the last unit in active service, the 685.222, was withdrawn in early 1975.
Conversions
Rebuilds from the 680 class
The by then-obsolete FS Class 680 locomotives were first subjected to a first rebuild, in which 91 of them were superheated but kept the compound engine ; after it was determined that these locomotives were still inferior in terms of performance and water and coal consumption, it was decided to rebuild them into Class 685 locomotives, instead. 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 ; they resulted even more powerful than the original Class 685s, thanks to their higher boiler pressure. A further group of 40 locomotives, with the new classification of 685.5XX, also received the Caprotti valve gear, and had a boiler that achieved even higher steam temperatures, to further improve their efficiency; some even received an exhaust injector.
The last batch of rebuilds, intended to serve on major lines on which the original axle load limits had been rescinded, were therefore subjected to an increase of their adhesive weight to improve performance, bringing their axle load up to 17 tons, and, taking advantage of the more relaxed weight limits, were fitted with a Knorr pre-heater; some of them also received a larger tender with a total water capacity of, used by the FS Class 691. In total, 49 were rebuilt until 1933, while in 1934 a 685.500, was further converted into a S.685. Based on their increased performance, they were classified as the separate Class S.685. The Class S.685 locomotives mainly operated on the Milan-Venice, Milan-Turin and Rome-Naples mainlines, especially on the former together with the FS Class 691. After these lines were electrified, respectively in 1958 and 1961, their increased axle load and maintenance needs led to their withdrawal by 1968. In total, 120 ex-Class 680 locomotives were eventually rebuilt, until 1933, bringing the total number of locomotives in the Class to 391.
Franco-Crosti
In 1939 five locomotives were rebuilt with Franco-Crosti boilers; initially classified into the Class S.685, in 1951 they were reclassified as FS Class 683.
Turbine locomotive
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.
Preservation
Five Class 685 locomotives survive into preservation:
