RS4 Codici

The term is an abbreviation of Ripetizione Segnali a 4 codici (signal repetition system with 4 codes).

It helps the driver to reliably determine the aspect of the next signal even in poor visibility and at high velocities.

The information is conveyed via amplitude modulation of a 50 Hz alternating current of the Blocco Automatico a Correnti Codificate (BAcc) in the rails.

Receiver coils in front of the first axle of a locomotive or control car are used to detect the signal.

As trains got faster an additional set of codes sent with a 178 Hz carrier were added.