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.

[1] The English author Peter Michael Kalla-Bishop claims 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run, a prototype (numbered FS 6901) was built with a number of different features, but was so unsuccessful that it was withdrawn and quietly scrapped after just a few months.

The firebox had to be placed between the rearmost driving wheels and was therefore of trapezoidal shape and relatively small; this feature would prove to be the locomotive's weakness, as it caused poor steaming and was expensive to maintain.

[3][2][4] Limited by their high axle load to the Milan-Venice and Milan-Bologna-Florence mainlines, the Class 690 did not prove to be satisfactory, as its poor steaming qualities greatly hampered the performance.

[3][5] As a result, by the end of the 1920s a new three-cylinder Pacific locomotive (to be classified as Class 695) was designed; however, although more powerful than the Class 690, it would have required considerable strengthening of bridges and other infrastructures of the railways it was supposed to work on, because of an axle load of 21 tons.