They were, as another poster stated 3-phase AC locomotives. Mechanically, they were based on developments by the Hungarian engineer Kandò.

The big advantage of that electrification was "free" regenerative braking, and pretty strict holding to the speed given by the number of active poles in the motors. Disadvantage was acceleration, which had to be dealt with resistors. Some models had salt-based liquid resistors.

The changeover from 3-phase AC to 3 kV DC happened all at once for a specific line. In some cases, they kept the overhead wires, but moved them close together towards the middle, and so had a double wire for the DC (in the case of the Valtellina, that happened).

The last pieces of that network got converted in the mid-1970s in the Piemonte.

Nowadays, I can recount three lines where 3-phase AC electrification is still active:

• Jungfraubahn (Swiss Alps, near Grindelwald/Lauterbrunnen/Gimmelwald)
• Gornergratbahn (Swiss Alps, Zermatt)
• Chemin de Fer de la Rhune (Pyrenées)

All three lines are cog railways, where the above mentioned advantage beat all its disadvantages with the complex overhead wiring etc. However, the very newest trains on the Swiss lines have a 3-phase fixed frequency --> DC --> 3-phase variable frequency converter, which allows higher speeds.