[1][2] Previous fast locomotives were of the 4-4-0 arrangement; Frescot decided to add one more driving axle to ensure that the locomotive would be able to pull the increased loads without exceeding the axle load limits.

The first locomotives had a working boiler pressure of 10 kilograms per square centimetre (981 kPa; 142 psi), while subsequent locomotives enjoyed a greater pressure of 11 kilograms per square centimetre (1,080 kPa; 156 psi); three locomotives were also fitted a longer boiler with a combustion chamber.

[5] The first locomotive was the only one given a running number by the Società per le strade ferrate dell'Alta Italia, and was also the only one given a name ("Vittorio Emanuele II", in honour of King Victor Emmanuel II of Italy), as just as it was completed the transition between the SFAI and the Rete Mediterranea was being executed.

The Class 650 proved very successful, being able to pull 130 tons over the new Giovi railway at a speed of 45 kilometres per hour (28 mph).

However, the steady increasing of the train's weight meant that these locomotives were gradually replaced by more powerful ones.