FS Class 685

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 (with some of them receiving a larger high pressure (HP) cylinder); 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.[12][13]

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 (1,570 kPa; 228 psi); they resulted in being 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.

FS Class S.685
FS locomotive S.685.600 as preserved
Type and origin Power type Steam Total produced 50 Rebuilder Ernesto Breda, Officine Meccaniche, FS Works of Pietrarsa Rebuild date 1929-1934
Specifications Configuration: ​  • UIC 1'C1'4h Gauge 1,435 mm (4 ft 8+1⁄2 in) Leading dia. 960 mm (3 ft 2 in) Driver dia. 1,850 mm (6 ft 1 in) Trailing dia. 1,220 mm (4 ft 0 in) Length 11,730 mm (38 ft 6 in) Axle load 17 tonnes (17 long tons; 19 short tons) Loco weight 73.4 tonnes (72.2 long tons; 80.9 short tons) Tender weight 54.1 tonnes (53.2 long tons; 59.6 short tons) Fuel type Coal Fuel capacity 5,500 kilograms (12,100 lb) Water cap. 29,000 litres (7,700 US gal) Firebox:  • Firegrate area 3.5 square metres (38 sq ft) Boiler pressure 16 kilograms per square centimetre (230 psi) Heating surface 188.74 square metres (2,031.6 sq ft) Cylinders 4, simple Cylinder size 420 mm × 650 mm (16.54 in × 25.59 in) Valve gear Caprotti
Performance figures Maximum speed 120 kilometres per hour (75 mph) Power output 1,350 CV (990 kW; 1,330 hp) Tractive effort 11,800 kilograms-force (116 kN)

The last batch of rebuilds, intended to serve on major lines on which the original axle load limits had been rescinded, were therefore subjected to an increase of their adhesive weight (by shifting the pivots of the Italian bogie and the trailing axle towards the driving axles) 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 29,000 L (7,700 US gal), 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 (comparable or, in some cases, even superior to the larger FS Class 690 Pacifics), they were classified as the separate Class S.685 ('S' standing for Speciale).

The Class S.685 locomotives mainly operated on the Milan-Venice, Milan-Turin and Rome-Naples (through Cassino) 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.[14]

In total, 120 ex-Class 680 locomotives were eventually rebuilt, until 1933, bringing the total number of locomotives in the Class to 391.[15][16][17]

In 1939 five locomotives (amongst those fitted with Caprotti valve gear) were rebuilt with Franco-Crosti boilers; initially classified into the Class S.685, in 1951 they were reclassified as FS Class 683.[18][19][20]

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.[21][22][23]