Blocco automatico a correnti codificate (BACC or BAcc, ''automatic block with codified currents'') is a

signalling block system Signalling block systems enable the safe and efficient operation of railways by preventing collisions between trains. The basic principle is that a track is broken up into a series of sections or "blocks". Only one train may occupy a block at a ...

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 re ...

on railway lines using 3 kV DC electrification. The

track circuit A track circuit is an electrical device used to prove the absence of a train on rail tracks to signallers and control relevant signals. An alternative to track circuits are axle counters. Principles and operation The basic principle behind t ...

s used to detect the presence of a train also transmit coded signals to the trains which are used for train protection and

cab signaling 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 ...

. Train protection systems that use BAcc are

RS4 Codici RS4 Codici is a train protection system used in Italy. The term is an abbreviation of ''Ripetizione Segnali a 4 codici'' (''signal repetition system with 4 codes''). It is a simple cab signalling system, displaying the aspect of the next (and, in ...

, RS9 Codici and

SCMT Sistema di Controllo della Marcia del Treno (SCMT) is a discontinuous train cab signalling system used in Italy. It shares many features with the Ripetizione Segnali (RS) system, the two systems co-existing and working together. The main pur ...

Codes

The information is conveyed by superposition of two amplitude-modulated

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 whic ...

s in the rails (with a

carrier frequency In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a m ...

of 50 Hz and 178 Hz, respectively). Receiver coils in front of the first axle of a

locomotive A locomotive or engine is a rail transport vehicle that provides the Power (physics), motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, Motor coach (rail), motor ...

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: {, class="wikitable" , - ! modulation frequency of 50 or 83.3 Hz carrier , , modulation frequency of 178 Hz carrier , , code , , meaning , - , (none) , , (none) , , AC
(''Assenza di Codice'') , , transmission failure, e.g. on a siding not equipped with the required track circuitry , - , 75 / min , , (none) , , 75 , , 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 , , (none) , , 120 , , the next signal displays a speed restriction (to 30 km/h, 60 km/h or 100 km/h) , - , 120 / min , , 75 / min , , 120* , , the next signal displays a speed restriction to 100 km/h , - , 120 / min , , 120 / min , , 120** , , the next signal displays a speed restriction to 130 km/h (this code is currently not being used) , - , 180 / min , , (none) , , 180 , , the next signal is at "proceed", but the signal after that is at "stop" or displays a speed restriction (this code is also being used to announce the end of the BACC-equipped line) , - , 180 / min , , 75 / min , , 180* , , the next signal is at "proceed", but the signal after that displays a speed restriction (to 100 km/h or 130 km/h), or the train is approaching a

slow zone In the United States, a slow zone is an area where a train is forced to slow down for either structural, construction, power, signal, or track problems. Slow zones limit a train to about . Notification to train crews is referred to as a "slow ord ...

with a speed limit of 150 km/h or less , - , 270 / min , , (none) , , 270 , , proceed with a maximum speed of 180 km/h , - , 270 / min , , 75 / min , , 270* , , proceed with a maximum speed of 220 km/h , - , 270 / min , , 120 / min , , 270** , , proceed with a maximum speed of 250 km/h While RS4 Codici 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 that only requires the driver to acknowledge any change in the aspect of the next

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'' ...

, SSB (On board sub system) RS9 equipment also continuously monitors the train speed and computes braking curves according to the train's length, mass, and braking ability. RS4 Codici uses a single 50 Hz alternating current. All codes that do not use the 178 Hz carrier are used identically for both RS4 and RS9 Codici. Thus, RS9 is

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 i ...

to RS4 Codici. An alarm is sounded in the cab if the train exceeds the speed setpoint by more than 3 km/h. If the train exceeds the speed setpoint by more than 5 km/h or misses the designated stopping place, the system applies emergency brakes.

References

* https://web.archive.org/web/20060506191231/http://www.segnalifs.it/it/bl/N_barsc.htm Railway signalling in Italy Electric railways in Italy