With a geographical distribution complementary to that of class 835, these "interesting machines"[1] 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[note 2] of the titular machines especially on the steep pass lines.

Therefore tank locomotives[2] with good adherent mass, power and speed and sufficient range for those types of services were studied.

[note 4] 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).

[19] The opinion is shared by Giovanni Cornolò[20] and Gian Guido Turchi,[5] 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.

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,[note 11] with installation of the compressor (single stage) on the right side.

The three machines taken over (unit 02 had been damaged during the World War and was scrapped in 1947)[31][32] 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).

[29][30][32][33] 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.

[36][37] 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.

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.

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.

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.

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.

[note 23] In 2013, the situation was as follows:[58] 851,036 at Laghi di Sibari; 043 owned by the Associazione Amici della Ferrovia Suzzara-Ferrara in San Benedetto Po; 851.