The Class 690 was designed just as the superheater technology was becoming available, allowing the FS to discard what had been a widespread feature on many Italian locomotives, the compound engine, deeming the advantages of the simpler single-expansion engine coupled with superheated steam to be superior.
The English author Peter Michael Kalla-Bishop said that, just before the production run, a prototype (numbered FS 6901) was built with a number of different features, but was so unsuccessful that it was withdrawn and quietly scrapped after just a few months. However, there is no reference for this, which is not repeated in any other document.
The first nine locomotives, all with right-hand drive, were built in 1911, six by Ernesto Breda and three by the Officine Meccaniche; the following twenty-four, fourteen of which were built in 1914 by Breda and ten by Gio. Ansaldo & C., had left-hand drive. They had all been designed for an axle load of 18 tons, but since this value was too high for even the current mainline railways, they all entered service with the load on the driving wheels lightened to 17.1 tons. They were the first Italian four-cylinder simple-expansion locomotives that had the adjacent cylinders paired together, each pair served by a single piston valve through crossed ports (a feature shared by the more numerous Class 685). The firebox had to be placed between the rearmost driving wheels, and was consequently trapezoidal and relatively small, causing poor steaming and high costs; this was a significant weakness of the locomotive.
