FS Class 690 II

Ferrovie dello Stato Italiane

Industry	Railway transport
Founded	1905
Headquarters	Rome , Italy
Area served	Italy

For the initial use (1905–1907) of FS Class 690, see FS Class 670.

Template:Infobox locomotive The Class 690 (Italian: Gruppo 690) was a 4-6-2 'Pacific' steam locomotive model operated by the Ferrovie dello Stato Italiane (FS; Italian State Railways) and intended for high-speed passenger service.

Design and construction

The development of the Class 690 coincided with the introduction of superheater technology, which led FS to abandon the previously common use of compound locomotive engines in Italy. The railroad deemed that the simpler single-expansion engine combined with superheated steam offered superior performance.Template:Sfnb

According to British author Peter Michael Kalla-Bishop, a prototype numbered FS 6901 was constructed shortly before full production began, featuring several experimental elements; however, it reportedly failed to meet expectations and was withdrawn and dismantled within months. This claim, however, lacks corroborating evidence and is not mentioned in other sources.Template:Sfnb

The initial batch of nine locomotives, all equipped with right-hand drive, was produced in 1911—six units by Ernesto Breda and three by Officine Meccaniche. Subsequently, twenty-four more were built: fourteen in 1914 by Breda and ten by Gio. Ansaldo & C., all featuring left-hand drive. Although originally designed for an axle load of 18 tons, this was too heavy for the mainline infrastructure at the time; therefore, the axle load on the driving wheels was reduced to 17.1 tons before entering service. These were Italy’s first four-cylinder simple-expansion locomotives with adjacent cylinders paired together, each pair operated by a single piston valve via crossed ports—a design also adopted on the more numerous FS Class 685. The firebox was positioned between the rear driving wheels, resulting in a trapezoidal shape and relatively limited size, which impaired steaming capacity and increased operational costs; this represented a notable shortcoming of the class.Template:SfnbTemplate:SfnbTemplate:Sfnb

Operations

The Class 690 locomotives were restricted to operating primarily on the Milan-Venice and Milan-Bologna-Florence main routes due to their substantial axle load. Their overall effectiveness was limited because of inadequate steaming capacity, which significantly reduced operational efficiency. Although these engines were among the fastest in Italy—being the first to receive official certification for a maximum mainline speed of 130 km/h (81 mph)—and possessed considerable raw power, their relative performance was somewhat underwhelming. Specifically, they delivered 1400 CV at 90 km/h (1,030 kW; 1,380 hp at 56 mph), whereas the smaller Class 685, equipped with a shorter boiler, produced 1250 CV at 75 km/h (920 kW; 1,230 hp at 47 mph).Template:SfnbTemplate:Sfnb

By the late 1920s, a new three-cylinder Pacific locomotive, designated as Class 695, was developed to succeed the Class 690. Despite its increased power, its axle load of 21 tons necessitated substantial reinforcement of bridges and other railway infrastructure along its intended routes. Given that much of the Italian rail network was undergoing electrification or had already been upgraded, undertaking such costly modifications was deemed impractical. Consequently, a more conservative strategy was adopted: the existing Class 690 units were rebuilt by installing boilers and fireboxes from the FS Class 746. Between 1928 and 1934, all thirty-three locomotives underwent this transformation, emerging as the new Class 691.Template:SfnbTemplate:Sfnb

References

• Template:Kalla-Bishop-FS-Steam