This class, which should have been the Class 695, would have had however a very high axle load (as much as 21 tonnes (21 long tons; 23 short tons)), which would have required strengthening of the lines they were supposed to serve; as electrification was becoming more and more prominent, such an expense was considered unwarranted.

Therefore, a less ambitious plan, which involved rebuilding the Class 690 locomotives (and which would have entailed a lower axle load), was approved.

The Class 691 served on the Milan-Bologna (up to 1938, when the line was electrified) and the Milan-Venice mainlines throughout their careers, pulling the heaviest and fastest express trains.

After the electrification of the Milan-Venice railway in the spring of 1957, some locomotives remained active for some time on the Venice-Cervignano del Friuli line, but by the early 1960s they were all withdrawn.

[3] The 691.011 scored the speed record, between the stations of Verona and Padova, for the Italian steam locomotives, with 150 kilometres per hour (93 mph).