In the early 1870s, the SFAI needed a locomotive suitable for heavy work on the most important mountain lines, such as the Giovi railway and the Turin-Modane railway, for which the 0-6-0 locomotives were becoming increasingly inadequate. The Ufficio d'Arte di Torino chose a 0-8-0 locomotive of the Wiener Neustädter Lokomotivfabrik (then known as "Sigl"), very similar to the Südbahn Class 35 a that it already produced.
The Class 420 was a typical long-boiler, inside-frame 0-8-0 locomotive of the era, that showed its Austrian derivation with its two-shutters smokebox door, and its outside Stephenson valve gear. The locomotives built before 1884 had the distinction of having curved foot plating over the wheels, while later units had straight foot plating and small splashers. Some of the locomotives were given a replacement boiler before 1914, but their performance remained mostly unchanged.
The first 60 locomotives were built by Sigl (from which they derived the nickname with which they were known for their whole career) for the SFAI. Production continued until 1890, from both foreign (such as Maffei) and Italian firms (such as Ansaldo and Breda), for a total of 189 locomotives; all these were divided in 1885 between the Rete Adriatica and the Rete Mediterranea. Building of further locomotives for the RM resumed in 1897, and continued until 1905, bringing the total of the Class to 293.
