Class 851 locomotives were a class of steam locomotives of the Italian State Railways (FS).
 They were designed and built by the Adriatic Network (RA) as machines for line service. In 1905, together with the locomotives of the classes FS 290, 600 and 870 ex-RA, they were included among those that the FS deemed worthy of further orders pending the completion of the designs of new classes suitable to cope with the development of passenger and freight traffic resulting from nationalization.
 The advent of Class 940 locomotives relegated them to the role of shunting locomotives. With a geographical distribution complementary to that of class 835, these "interesting machines" served until the end of the steam traction era in Italy in the 1970s.
 At the end of the nineteenth century, the completion of the Sulmona-Isernia and Candela-Potenza lines and the increase in train compositions and passenger car masses determined the management of the Adriatic Network to the design of a new group of steam locomotives intended to support, by placing at the head or rear of the trainsets, the tractive effort of the titular machines especially on the steep pass lines. Therefore tank locomotives with good adherent mass, power and speed and sufficient range for those types of services were studied.
 Incorporated into the new RA 270 class and delivered in 1898 in an initial subgroup of 18 units, they were destined for the aforementioned Sulmona-Isernia and Candela-Potenza lines and the Terni-Ancona section of the Rome-Ancona and Pescara-Avezzano section of the Rome-Pescara.
 Good operating results prompted the construction of a second subgroup in two separate lots of 18 and 24 units, which were delivered between 1900–01 and 1904.
 The FS, which took over from the previous private companies in 1905, had to quickly set up plans for the development and renewal of their fleet. While the Study and Testing Office of the Traction Service was designing new construction types while also taking up earlier studies, it was decided to commission more units of the pre-existing types possibly with modifications suggested by the practice of operation (FS classes 290, 320, 600, 630, 670, 750, 830, 851, 870, 904 and 910; in addition, they wanted to test U.S. construction techniques by ordering the 20 machines of classes 666 and 720). Among them, former RA 270s were chosen, which formed the 851 FS class. Construction continued until 1911 for a total of 207 units, which from 1917 were numbered 851,001-207.
 The design of RA 270 was developed by the Florence Study Office of the Adriatic Network and in some details shows a relationship to the contemporary 350 bis RA class, later FS 290.
 As typical of the "Florentine school," simplicity of design was favored, especially of the engine and distribution mechanism. This choice generated significant economy of operation, which also had among its consequences a marked appreciation by the engine and shop personnel.
 According to engineer Bruno Bonazzelli the RA 270, later named FS 851, can be considered the improved version of the RA 250, later named FS 827 class locomotives. The opinion is shared by Giovanni Cornolò and Gian Guido Turchi, who point out the common destination for reinforcement service, at the head or tail end of trains, on the steepest ramps of trans-Apennine lines.
 For the services expected to be performed on mountain lines with gradients of up to 28 per thousand and numerous trunks armed with rails having a mass of 27 kilograms per metre (54 lb/yd), a running gear (UIC) C was chosen with wheels of 1,510 mm (59 in) diameter, spacing between extreme wheelsets of 4,000 mm (13 ft), all adhering mass, and a modest load per wheelset (14.3 and 14.4 t [14.1 and 14.2 long tons; 15.8 and 15.9 short tons] in the first subgroup and 14.6 and 14.7 t [14.4 and 14.5 long tons; 16.1 and 16.2 short tons] in the second subgroup).
 In the machines of the second subgroup, the consideration of the possibility of frequent stops for refueling prompted the reduction of water stocks from 5,700 to 5,000 L (1,300 to 1,100 imp gal; 1,500 to 1,300 US gal) and coal stocks from 1.4 to 1.2 t (1.4 to 1.2 long tons; 1.5 to 1.3 short tons) (the increase in overall mass was due to the toughening of the chassis).
 The steam generator was of the normal type in use at the time of design. The combustor had a grate with a surface area of 1.53 m2 (16.5 sq ft). The cylindrical body of the boiler was 4.41 m (14.5 ft) long (including the smoke chamber) and contained 213 kettle tubes, 3.2 m (10 ft) long, between the furnace plate and the smoke chamber plate.
 The boiler, which contained 2.9 m3 (640 imp gal; 770 US gal) of water, had a heating surface of 94.16 m2 (1,013.5 sq ft), including 87.8 m2 (945 sq ft) in the kettle tubes and 6.36 m2 (68.5 sq ft) in the part of the furnace above the grate ("furnace ceiling"). It generated saturated vapor with an output of 4,700 kg (10,400 lb) per hour.
 The engine was single-expansion, with twin outer cylinders having a bore of 430 mm (17 in) and piston stroke of 580 mm (23 in), Walschaerts mechanism, and slide valves.
 Like the FS 290 locomotives, all 851s were equipped with a variable escapement of the "pear" type.
 The normal power was 294 kW (394 hp) at 30 km/h (19 mph) (this explains the later use in shunting: in steam traction the power, and thus the load that can be pulled, is maximum in the middle range of developable speeds).
 The first subgroup was equipped with the vacuum brake. Later, in compliance with legislative requirements, first the newly built machines and then the others were equipped with the Westinghouse type automatic and proportional air brake, with installation of the compressor (single stage) on the right side.
 All machines were arranged to deliver steam for heating passenger cars.
 The machines of the first delivery had an open cab on the rear side, then enclosed with a wall equipped with three large windows, the middle one of which could be opened to increase ventilation in the cab.
 The paint schemes adopted were those normally used first by the RA and then by the FS. The scheme, in gray, used provisionally for photographic shots for the purpose of documenting the activities of the manufacturing industries was applied to some.
 The performance assigned by the FS Material and Traction Service and published in the General Preface to the Service Schedule (PGOS) was the same for classes 851 and 981 (source:):
 The values are expressed in tons.
 The locomotives were built from 1898 to 1911 in a total of 207 units. Numbered first RA 2701-2760, then FS 8511-8699 and 85290-86307, from 1917 they assumed the final numbering FS 851.001-207, practically applied in 1919 (former RA 2732-2755 became FS 851. 001-024; former RA 2701-2712, 2718-2731, 2713–2717 and 2756-2760, following the order of delivery, became FS 851.079-114).
 Four other machines corresponding to the 851 class were built in 1924 by Ansaldo for the Siena-Buonconvento-Monte Antico railway, which was bought back by the FS in 1956. The three machines taken over (unit 02 had been damaged during the World War and was scrapped in 1947) had the numbering 851.01 and 03-04 and retained it until their decommissioning that took place between 1958 and 1961 (Peter Michael Kalla-Bishop mentions the possibility, which did not materialize, of their renumbering as FS 851.208-210).
 In addition, the Società Anonima Strade Ferrate Sovvenzionate (SFS), operating the Caudina Valley Railway (Benevento-Cancello line), had six locomotives identical to FS 851 built by the Officine Meccaniche in 1911.
 The FS divided its 851s into two subgroups: the first included those built in 1898 which, in 1917, received the new numbering 851.079-090 and 105-109 and the second all others.
 The breakdown of units among the manufacturers and the sequence of numbering is in the following table. The numbering of former RA 2701-2772 was maintained by the reconstituted Strade Ferrate Meridionali (SFM), which took over the operation of the Adriatic Network between 1903 and 1906 pending its nationalization in 1906.
 Units 851,139-158 were built by Officine Meccaniche in their Naples plant.
 According to Briano in 1906 there were 72 units in service out of a total of 409 tank locomotives in the FS fleet. Gian Guido Turchi, on the other hand, writes that in 1906 there were 88 units in operation, plus another 26 under construction.
 Unlike the other rolling stock C locomotives that came to the FS in 1905-1906) the 851s, designed for reinforcement services on mountain pass lines, proved sufficiently powerful, fast and versatile to be able to perform line services with direct passenger trains as well. Therefore, the FS stipulated that, during major repairs, their boilers be replaced with those of the 875 class, designed in 1911 for hauling freight and passenger trains on secondary lines with little traffic.
 The fitting on 851.181 of a boiler of 870, carried out in the Officina Grandi Riparazioni at Pietrarsa in March 1930, was probably due to contingent needs rather than experimentation.
 In addition to replacing the vacuum brake with an air brake and replacing the boiler, the most important modification undergone by some 851s was the provision for mixed combustion (naphtha and coal). Inspired by the practice of operating with the class 736 locomotives left in Italy by the United States Transportation Corps, which were fueled by naphtha, it was designed to reduce coal consumption (naphtha was used for normal service and coal for ignition and standing fire). Between 1947 and 1948 at least 122 locomotives of various classes (835, R.302 and 27 units of class 851) were thus modified by the OGR in Verona Porta Vescovo and the Officina Grandi Riparazioni in Rimini. They were deployed mainly in the classification yards of Naples, Bologna, Reggio di Calabria, Bari, Rome and Parma, and arrived almost all of them with this modification until they were scrapped.
 The 851s performed the services for which they were designed until the 1920s. Then the increase in train compositions and the introduction into the fleet of an increasing number of metal box coaches, together with that of locomotives of class 940, divided between the Sulmona and Campobasso depots, made it possible to withdraw the 851s from many line services (in some of which they had to be engaged in double and triple traction because of the increase in the masses to be hauled) and to assign them to shunting services, especially in central and southern Italy. In them they had the opportunity to make their good power and top speed characteristics appreciated, which, owing to the all adhering mass, allowed the delivery of significant performance, better than that of the 835s, even in the typical operating regime of shunting.
 Among the last line services were those towing local passenger trains performed on the Rovigo-Chioggia line until about 1969.
 Locomotives 851.011, 040, 121 were leased to the Ministry of the Navy from 1939 until an unknown date.
 Locomotives were sold: 851.006 to the Ministry of the Navy in 1935, 037 to Breda in 1939, 181 and 187 to the Italian Company for Southern Railways in 1930. Gian Guido Turchi added to the previous units the 150, sold to Railway Connections of Marghera in 1943.
 The longevity of the class, due to its qualities, favored advocates of preserving some of its units.
 Locomotive 851.110, restored by the workshop of the Bologna Centrale locomotive depot, is preserved in the Pietrarsa National Railway Museum. Others are stored with private individuals or in public gardens.
 In 1992 the situation was as follows:
 851. 043 in consignment to the Centro per la storia delle ferrovie e tramvie, Bologna; 057 in Osnago; 066 at the Impresa Furlanis, Fossalta di Portogruaro; 074 owned by the Associazione Treni Storici Emilia-Romagna-Adriavapore in Rimini; 103 in the Bersano Museum in Nizza Monferrato; 105 in Faenza; 112 in Mestre, home of the Gruppo Fermodellistico Mestrino; 113 in Bitonto; 130 in Rapallo; 186 in Como; 203 in Bologna Centrale awaiting transfer to Guastalla for monumentation.
 In 2013, the situation was as follows:
 851,036 at Laghi di Sibari; 043 owned by the Associazione Amici della Ferrovia Suzzara-Ferrara in San Benedetto Po; 851. 057 owned by the Associazione Gruppo ALe 883 in Tirano; 066 owned by the Impresa Furlanis in Fossalta di Portogruaro; 074 owned by the Associazione Treni Storici Emilia-Romagna-Adriavapore in Rimini; 097 in Ponton di Domegliara; 103 in Nizza Monferrato; 105 in Faenza; 112 in Mestre; 113 in Palagianello; 130 in Rapallo; 186 in Como; and 203 in Novafeltria.
 In 2016 the situation was unchanged, with the exception of units 036, demolished in early 2015, and 203, moved to Perticara.
 Models of the 851 FS locomotives have been produced by several specialized firms including Rivarossi since 1964. Completely new models in H0 scale were brought to market in 2016 by the firms LE Models, Hornby (under the Lima brand) and OsKar.