They were designed to fulfill a requirement from the Ferrovie dello Stato (or FS, then the quasi-monopolist of Italian railways) for a new locomotive to be used with medium-weight passenger trains and a similar one for cargo services on steep lines.

The power plant is made of three 1,635 kilowatts (2,193 hp) FS T850 DC motors, mounted on monomotor bogies, each with two axles.

Each motor is provided with a three-frequency chopper which takes place of the resistor network used on older Italian locomotives (rheostat).

Since the beginning, all units were provided with the standard 78-wire cable for coupling with driving passenger cars, useful in the formation of commuter push-pull trains.

40 E633s were also fitted with a 13-wire ZDS cable (comando multiplo) in order to allow hauling with multiple units by only one engine crew, from the "master" locomotive.