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The Ferrovie dello Stato Italiane (FS; Italian State Railways) Class 746 (Italian: Gruppo 746) is a class of 2-8-2 'Mikado' steam locomotive. These were the biggest locomotives ever built by the Ferrovie dello Stato, even though not the fastest, which were the Class 691s.
The Class 746 originated from the need of a fast locomotive that, while having similar performance to the 'Pacific' Class 690, could have a lower axle load and therefore wider route availability; this could be achieved by adding another driving axle, and accepting a lower top speed (reverting to a leading Italian bogie). To achieve high performance, it was decided to adopt a symmetric compound engine (a Von Borries design), with the two inside high pressure (HP) cylinders having the motion for their valves derived from the outside Walschaerts valve gear; this was unusual for Italian locomotive practice, where the compound engine had been shelved once the superheating technology had become available in the 1910s. [1] [2]
Forty locomotives were ordered to the Ernesto Breda, which produced them in 1922, and then ten more in 1926; Gio. Ansaldo & C. built ten more, fitted with Caprotti valve gear, in the same years. The latter were initially classified as Class 747, but in 1930 they were reclassified as the 746.100-110 subclass. [2]
The Class 746 initially operated on the Milan-Bologna-Florence-Rome-Naples main line; after portions of it were electrified, they served on the Milan-Turin and the Milan-Brescia railways. In the 1960s, they were operating in Sicily and on the Adriatic coast railway (based at Ancona shed), until their withdrawals, which were concluded by 1967. While fast, powerful and suitable for both fast passenger and goods trains, the Class 746 proved difficult to start and somewhat uneasy when running at or close to its top speed (with both problems originating from its compound engine); also, at higher speeds the low-pressure cylinders contributed relatively little to the locomotive's power output. The locomotives fitted with Caprotti valve gear also turned out to have problems when functioning in reverse. [2] [3] Their poor riding qualities and their huge mass (for Italian standards) led some Milanese crews to nickname them "Krassin" [lower-alpha 1] . [4]
In 1930, all the locomotives were modified with improved draughting. In 1932, four locomotives, two with Walschaerts valve gear (the 746.007 and the 046) and two with Caprotti valve gear (the 746.107 and the 108), were modified with a simple-expansion engine; while results were very encouraging, with the modified locomotives being as performant as the unmodified ones but performing better at higher speeds, the widespread electrification meant that this experiment was not followed up with more conversions. [2] [4]
Two Class 746 locomotives survive into preservation: the 746.031, a static exhibit in the Museo della Scienza e della Tecnologia "Leonardo da Vinci" in Milano, and the 746.038, long preserved in the FS workshop of Verona and recently moved to Pistoia, to await a possible restoration.
The Caprotti valve gear is a type of steam engine valve gear invented in the early 1920s by Italian architect and engineer Arturo Caprotti. It uses camshafts and poppet valves rather than the piston valves used in other valve gear. While basing his design on automotive valves, Caprotti made several significant departures from this design to adapt the valves for steam. Having agreed a joint-venture with Worcester-based engineering company Heenan & Froude from 1938, Heenan & Froude fully acquired Caprotti post-World War II in 1947.
The Ferrovie dello Stato Class 625 is a class of 2-6-0 'mogul' steam locomotives in Italy. The class is commonly known by the nickname Signorine, or Signorina in the singular, because of their perceived grace and beauty compared to other locomotives.
The Ferrovie dello Stato Class 740 is a class of 2-8-0 'Consolidation' steam locomotives.
The Ferrovie dello Stato Class 685 is a class of 2-6-2 'Prairie' express train steam locomotives. These are colloquially known as Regine, mirroring their fame as one of the most successful and appreciated Italian steam locomotives.
The Ferrovie dello Stato Italiane Class 691 is a class of 4-6-2 'Pacific' locomotives; they were the fastest and most powerful locomotives ever built for the Italian railways.
The Ferrovie dello Stato Class 640 is a class of 2-6-0 'Mogul' steam locomotives in Italy. Commonly nicknamed "Signorine", a nickname shared with the similar Class 625, these locomotives were the first superheated steam locomotives in Italy.
The Rete Adriatica Class 500, classified after 1905 in the Ferrovie dello Stato Italiane as Class 670 was an unorthodox and iconic cab forward 4-6-0 (2'C) steam locomotive.
The Ferrovie dello Stato Italiane Class 690 was a 4-6-2 'Pacific' steam locomotive for express trains.
The Ferrovie dello Stato Italiane Class 470 is a 0-10-0 steam locomotive.
The Ferrovie dello Stato Class 480 is a 2-10-0 steam locomotive.
The Ferrovie dello Stato Italiane Class 680 is a 2-6-2 express steam locomotive; it was the direct ancestor of the very successful and appreciated Class 685.
The Ferrovie dello Stato Italiane Class 600, formerly Rete Adriatica Class 380 and Società per le Strade Ferrate Meridionali Class 380, is a 2-6-0 'Mogul' steam locomotive; it is considered by some as the first Italian modern steam locomotive.
The Ferrovie dello Stato Italiane Class 835 is a 0-6-0T steam locomotive; it was the standard steam shunter of the FS.
The Ferrovie dello Stato Italiane Class 290, formerly the Rete Adriatica Class 350 bis, is a 0-6-0 steam locomotive.
The Ferrovie dello Stato Italiane Class 420, formerly SFAI Class 1200, Rete Adriatica Class 420 and Rete Mediterranea Class 480, was a 0-8-0 steam locomotive.
The Ferrovie dello Stato Italiane Class 310, formerly Rete Mediterranea Class 380, was a 0-6-0 steam locomotive; they were the first Italian as-built compound locomotives.
FS Class 410 were 0-8-0 steam locomotives which the Italian State Railways (FS) acquired from Rete Sicula on nationalization in 1906.
FS Class 910 were 2-6-2 tank locomotives designed by Guglielmo Cappa for Rete Sicula where they were registered as RS Class 400. They were absorbed by Italian State Railways (FS) in 1905 and became FS Class 910.
The Ferrovie dello Stato Class 745 was a class of 2-8-0 'Consolidation' steam locomotives.
The Ferrovie dello Stato Class 744 is a class of 2-8-0 steam locomotives; they were the last orthodox steam locomotives built for the FS.
