intermittent and continuous ATP 101119 - irse.nl

end was traditionally done means of tuned mechanical relays. The supervised speeddepends on the speed code, i.e. the modulation frequency at which the track circuitcurrent is switched on and off. These devices are simple, robust and used to be costeffective, as they have very low numbers of parts. There are also certaindisadvantages for this type of continuous ATP/ATC system namely:• The train with the worst braking characteristics defines the braking profile, faster trains withbetter brakes have to brake early.• The brake supervision is very coarse, due to the small amount of sped codes available, andusually a long overlap has to be provided (typically one block section).• The train always has to move from receiver to transmitter of the track circuit, bi-directionalrunning requires a switchover of transmitter and receiver.• High return currents and / or harmonics etc. generated by modern traction systems and allsorts of disruption in the “code transmission, e.g. at block joints, special trackwork etc. cancause corruption of the information as these systems do not employ any for of data protectionor redundancy.In more recent systems, the same principle is used, but upgraded to increase theamount of information transmitted. In the Italian BACC system, a second carrier witha higher frequency is used to transmit additional information for high speed trains.The French TVM 300 and 430 systems use jointless track circuits and moreadvanced frequency modulation to transmit the information from track to train.TVM 430 layoutAll continuous systems described above transmit permissible maximum speedsto the train. The braking curve is based on one or two typical train types. Thesesystems were developed for lines with one or two types of trains only. Mixed trafficalways results in a non-optimal use of the line capacity.The German LZB system uses bi-directional data transmission through cableloops laid out between the rails. The system was specified by UIC and operates at acarrier frequency of 36 kHz to the trains and 56 kHz from the trains to the tracksideequipment. The data speeds are 1200 bit per second and 600 bit per second16