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 frequency | Code | Meaning |
| Assenza di Codice | Transmission failure | |
| 75 / min | Via Impedita | The 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 / min | Riduzione di Velocità | The next signal displays a speed restriction. |
| 180 / min | Preavvisa | The next signal is at "proceed", but the signal after that is at "stop" or displays a speed restriction. |
| 270 / min | Via Libera | Proceed 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 frequency | 178 Hz modulation frequency | Code | Meaning |
| 120 / min | 75 / min | Riduzione di Velocità 100 | The next signal displays a speed restriction of 100 km/h. |
| 120 / min | 120 / min | Riduzione di Velocità 130 | The next signal displays a speed restriction of 130 km/h. |
| 180 / min | 75 / min | Preavvisa 150 | The next signal is at "proceed", but requiring reduction in speed to 150 km/h. |
| 270 / min | 75 / min | Via Libera 220 | Proceed without restrictions at speed up to 220 km/h. |
| 270 / min | 120 / min | Via Libera 250 | Proceed without restrictions at speed up to 250 km/h. |