RS4 Codici


RS4 Codici is a train protection system used in Italy. The term is an abbreviation of Ripetizione Segnali a 4 codici.
It is a simple cab signalling system, displaying the aspect of the next signal. 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 in the rails. Receiver coils in front of the first axle of a locomotive or control car are used to detect the signal.
The frequency of the modulating signal encodes the signal aspect:
Modulation frequencyCodeMeaning
Assenza di CodiceTransmission failure
75 / minVia ImpeditaThe next signal is at "stop" or the train is about to enter a dead-end track, a partially occupied track or a track with particular short braking distance.
120 / minRiduzione di VelocitàThe next signal displays a speed restriction.
180 / minPreavvisaThe next signal is at "proceed", but the signal after that is at "stop" or displays a speed restriction.
270 / minVia LiberaProceed without restrictions.

The driver must acknowledge a code change by pressing a button labeled RIC within three seconds, otherwise the system will apply emergency brakes.
As trains got faster an additional set of codes sent with a 178 Hz carrier were added. The new system was backward compatible with the old 4-code system as trains unable to receive the 178 Hz codes would be relegated to a slower speed. The five additional codes are as follows:
50 Hz modulation frequency178 Hz modulation frequencyCodeMeaning
120 / min75 / minRiduzione di Velocità 100The next signal displays a speed restriction of 100 km/h.
120 / min120 / minRiduzione di Velocità 130The next signal displays a speed restriction of 130 km/h.
180 / min75 / minPreavvisa 150The next signal is at "proceed", but requiring reduction in speed to 150 km/h.
270 / min75 / minVia Libera 220Proceed without restrictions at speed up to 220 km/h.
270 / min120 / minVia Libera 250Proceed without restrictions at speed up to 250 km/h.