RS4 Codici is a

train protection system A train protection system is a railway technical installation to ensure safe operation in the event of human error. Development Train stops The earliest systems were train stops, as still used by the New York City Subway, the Toronto subway, ...

used in

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...

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

cab signalling Cab signaling is a railway safety system that communicates track status and condition information to the cab, crew compartment or driver's compartment of a locomotive, railcar or multiple unit. The information is continually updated giving an ...

system, displaying the aspect of the next (and, in some cases, the next but one)

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

. 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 Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which ...

of the

Blocco Automatico a Correnti Codificate Blocco automatico a correnti codificate (BACC or BAcc, ''automatic block with codified currents'') is a signalling block system used in Italy on railway lines using 3 kV DC electrification. The track circuits used to detect the presence of a train ...

(BAcc) in the rails. Receiver coils in front of the first axle of a locomotive or

control car A control car, cab car (North America), control trailer, or driving trailer (UK and Ireland) is a non-powered rail vehicle from which a train can be operated. As dedicated vehicles or regular passenger cars, they have one or two driver compartm ...

are used to detect the signal. The frequency of the modulating signal encodes the signal aspect: 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 Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially ...

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:

References

Sources

* http://www.segnalifs.it/it/bl/N_barsc.htm {{Railwaysignalling Railway signalling in Italy Train protection systems