LZB an Example of a Continuous system

The LZB system was specified by UIC in the 1960s. A first implementation was built between Augsburg an Munich in 1965, where a maximum speed of 200 kph was demonstrated in passenger traffic. In 1972, a first line between Hamburg and Bremen was taken into operation, which was based on a 2-out-of-3 computer system. Since then, all high speed lines in Germany were equipped with this system. Spain and Austria also adopted the computer based LZB system and lines in both countries are in operation.

System Summary

In the LZB system, all fixed data (i.e. line geography, permanent speed restrictions) are stored in the LZB centre. The interlocking systems forward signal aspects, point settings and other element data to the control centre, the trains within the range of the system transmit their specific data, e.g. train length, train position, actual speed, etc.
From this data the control centre determines for each train the target values such as distance to stopping point which then are converted on board to a maximum permissible speed. Since signal settings for several block sections ahead are forwarded, the maximum speed can be increased and the maximum braking distance can be longer than the signal aspects of existing lineside signalling would allow.
5.1.2 Operation of the LZB System
In the LZB system stationary trackside information is combined with information from mobile train-borne equipment. The resulting information is transmitted to the vehicles via inductive line loops. An inductive loop is formed between the rails by means of a cable laid according to installation standard B3 (UIC Standard ORE A46).
The maximum logical loop length is 12.7 km. The conductors are transposed every 100 m for compensating electrical characteristics as well as for determining the physical positions of the trains. The on-board equipment senses the phase change when passing a transposition. The number of transpositions passed is counted.

The position of a train is transmitted to the control centre by indicating the number of 100 m-sections traversed by the front of the train (coarse positions). The count of coarse positions is dependent on the direction of travel of the train within the length of an inductive loop. A second, independent fine positioning procedure installed on the locomotive allows to transmit the position of the train within each 100 m-section with a quantisation of 12.5 m (fine position).
The train control commands computed by the control centre are transmitted to the corresponding LZB train via a remote feeding device, inductive line conductors and antennae, and are processed by the trainborne LZB unit and forwarded to a control panel as well as to the ATO system. Normally the nominal speed (V/nominal), the actual speed, the speed to be reached within a determined distance (target speed), and the distance to the target point are displayed.

The actual speed of the trains is continuously monitored by the on-board equipment. If it exceeds the nominal speed by a certain permissible value, emergency braking is automatically activated.

Command and Control Centre

The data available from the interlocking system (signal aspects, point settings, positions of level crossing barriers, etc.) are transmitted to the control centre. In the opposite direction, approach indications for level crossings are transmitted. A higher level dispatcher system (i.e. a district control centre) can supply dispatching and scheduling data and receive operating information such as speed and position of trains from the LZB command and control centres.
In addition data is exchanged with neighbouring command and control centres to transfer train information between two control centres in both directions. The data flow to the train-borne units as specified by the ORE is vital, i.e. the command and control centre transmits messages to the trains with commands determined by the current condition of the total system. An operator terminal in the control centre allows entry of data for track sections with temporary speed restrictions and output of operating or fault messages.

Computer System

A major feature of the LZB system is the use of off-the-shelf process computers for calculating train commands in the command and control centres. The following considerations suggest the use of commercially available computers:
• powerful performance
• flexibility in adapting programs to each system
• high reliability by the use of highly integrated components
• favourable price-to-performance ratio

Fail-safe commands are always computed by two different computers with different program versions. The two results are compared by external or internal comparators. For reasons of availability, a three-computer system is used in this case. If one processor of the three-processor system fails, fail-safe operation from the signalling point of view can be maintained with two computers. The real-time requirements for the LZB computer system are very stringent. This is the reason for the use of a three-computer system rather than a cold standby configuration. In case of an interrupted data transmission, all trains would automatically be stopped. If one of the active computers fails, the configuration is automatically changed without interrupting the data flow to the trains. A notification message on the operator terminal informs the maintenance personnel.
In this manner a three-computer system offers safety by means of tripling of computers. The final logic element, the output selector unit, is constructed according to fail safe techniques. A failure of any of its components results directly in a blocking of the data flow, inhibiting any erroneous output. If a train receives no messages from the LZB centre, the train-borne unit is automatically disconnected. The train continues to travel under the driver’s control at a reduced speed (max. 160 kph) in the conventional signalling system.
This Three Computer concept has been built accordingly for the train-borne equipment, based on microcomputer technology.
5.1.5 Data Communication
Data communication between the control centre and the trains is via inductive line conductors and train-board antennae. A control centre handles a maximum of sixteen inductive line loops. This is equivalent for example to a two track section of about 40 km length with 10 siding tracks.
In installations nowadays short loop systems are chosen to avoid any operational downgrading when a cable loop is damaged. The technique used with short loops involves transmitting the data from the parallel remote feeding devices to the Command and Control Centre using the LZB transmission frequencies, so that the parallel remote feeding device requires no converter, only an amplifier. Each feeding device supplies four loops of 300 meters each. Two loops for each track allow for a total length of 600 meters to be covered by one feeding device.

The information is transmitted to the trains at a rate of 1200 bit per second. At least one message per second is addressed to each train within the range of the control centre.
The transmission speed of the information from the train to the centre is 600 bit per second.
Normally the following data are transmitted from the interlocking systems to the control centre:
• signal aspects
• point settings
• positions of level crossing barriers
• emergency stop (controlled by the interlocking system).

Each change of these data, especially any change of the signals and the level crossing barriers, has to be forwarded to the computer with as little delay as possible. If the channels of the interlocking systems are utilised to full capacity, the message containing an information change arrives at the master station after a maximum delay of 0.4 seconds.
In the opposite direction all interlocking systems are addressed via a common channel with each system having a different address. Normally, transmissions contain level crossing approach messages, commands for setting signal marker lights, as well as fault and operating state data of the LZB Command and Control Centre.
The data exchange between adjacent centres mainly consists of data for transferring trains. This exchange is effected over data transmission systems using the transmission speed specified by the ORE.

Software for the LZB centre

Since high processing speed is required, all lists and programs needed for the operating procedures are memory-resident. External mass storage cannot be used since access times are too long.
The most important data lists are the track section list and the train list. The track section list contains the coded track geography with fixed data (positions of the line components, track grades, track sections with permanent speed limits, entry/exit points on the loops, system boundaries) and the variable data (settings of the line components, track sections with temporary speed limits, emergency stop controlled by train and by control tower) of the total system.
In contrary to the static track section list which is permanently coded, the train list is a purely dynamic list. The specific data of all trains within the system area are entered for each program in this train list, monitored and cancelled upon departure. The structure of the train list ensures that each LZB train knows the positions of the preceding or following train (list chaining).
The main task of these programs is the determination of the maximum permissible braking distance for each train within the system area. To calculate the information for a train, input data from interlocking systems and adjacent controllers as well as the last message sent by the train have to be processed. In addition, the messages for the interlocking systems and adjacent controllers are prepared, operator monitor input is processed and possible fault and operating indications are transmitted.

SELCAB an example of an Intermittent System

One of the most challenging requirements to an ATP system is not to reduce line capacity. That means that on lines where throughput is critical, an upgraded signal aspect has to be transmitted with a minimum delay in order to avoid unnecessary braking and longer headways. This can be achieved by a semi continuous transmission system. In SELCAB transmission is based on LZB principles as described above. The length of the loop can vary according to the headway requirements. The loop will always be laid out in rear of the signal, allowing the vehicle to pick up the information of the signal continuously when positioned over the loop. Also when the train is stopped at a signal at danger, data transmission is guaranteed and the information transmitted will allow the train to accelerate when the signal has cleared and at the same time prevent a vehicle to start against a signal at danger. For non critical lines carrying low traffic, the length of the loop can be restricted to a minimum given by the maximum train speed and minimum time required to transmit the necessary information.
The use of a standardised means of transmission also has the advantage of well proven reliability and safety of the transmission method and also of the equipment.

The main characteristics of this data transmission method specified in ORE A46 are:
• Telegram length 83 Bits
• Hamming distance of 4
• 70 user defined bits per telegram
• 1200 Baud transmission rate
• FSK modulation
• Carrier 36 kHz +/- 400 Hz
• Current 100 mA - 200 mA.

The information for the train is generated locally at the signal by sensing the signal aspect and combining it with track specific data. This information is then sent to the vehicle on the loop, which picks up the magnetic field, decodes the telegrams and reacts accordingly.
The main difference to the LZB system is:
• No central trackside processing centre, local generation of information.
• No continuous loop layout, only in rear of signals.
• No addressing of trains, as only one train can be over the loop at a time.
• No transmission from train to track.

Therefore, the selected system has a more decentralised structure and needs less infrastructure.
As the data transmission characteristics are identical for SELCAB and LZB, the LZB trainborne equipment can run over both systems. Therefore, an upgrade of the line capacity based on a moving block system is possible by upgrading the trainborne equipment for train to track transmission.

System Overview

An overview is given in the block diagram below. The system takes its signal information locally from the lineside signalling system. In case of the Chiltern line the source of information is SSI. This interface also decouples the ATP system from the signalling system. The Loop Electronic Unit (LEU) has a memory with fixed telegrams.
The number of sets of telegrams depends upon the number of signal aspects which can be shown by the individual signal. Each set of telegrams corresponds to one signal aspect. The selection of the appropriate set of telegrams is made by the information picked up from the signal. The serial telegrams are frequency modulated and transmitted to trains via the inductive loop. The length of the loop can vary between 5 metres and 300 metres depending on the operational requirements (headway) of the line. As in LZB, the loops have transpositions which are used for the position measurement on board and cancel the effect of electromagnetic coupling to the rails. The information is picked up from the magnetic field of the loop cable by two onboard antennae. The rear antenna serves as a phase reference for the detection of transposition on the loop.
The vehicle on-board controller (VOBC) itself is a vital processor system in checked redundancy configuration which does all the scanning of the inputs decoding of telegrams, calculation of the various braking curves, speed supervision interfacing the driver’s Interface Unit (DIU) and the interfaces to the vehicle.
The position measurement system is based on three independent sensors:
• detection of transpositions of the loop cable (antennae)
• measurement of wheel revolutions (tachogenerators)
• measurement of acceleration/deceleration (accelerometer).

The driver’s Interface Unit itself is processor based, it handles the information flow between the data entry terminal and the VOBC and also drives the speedometer.

Operational Requirements

SELCAB uses a vital system, the driver has to be informed about the main restrictions of the line ahead and the train is supervised by the system. It is essential that the driver still has the responsibility for the movement of the train, but in case of not reacting according to the lineside signals or to speed restrictions, the train will be braked automatically.
The main functions are:
• Monitoring of train speed in relation to line speed, permanent and temporary speed restrictions, based on train characteristics.
• Supervision of braking to speed restrictions imposed by track characteristics or by signalling.
• Indication of line and signal information to the driver (cab signalling).

Trackside Equipment

There are three main components:
• Signal Interface Unit
• Loop Electronics Unit
• Cable loop

The loop cable is fixed to the track by clips attached to the flange of the rail, to sleepers and by covers attached to the sleepers over each loop crossover, or loop end. The covers protect the loop from damage by people walking on the track, or from dragging equipment. The SELCAB track loops do not infringe the ballast tamping zone and therefore it is possible to carry out periodic track maintenance with the loops in place.
The Loop Electronics Unit (LEU) consists of two telegram generators which are independent of each other, feeding their information through selection logic to the modulator/transmitter. Each of the two telegram generators reads the signal information from the six input lines. These inputs form a dual code, therefore 26 = 64 different combinations can be defined. Each defined state selects a certain set of telegrams.
The information for selecting the correct set of telegrams is derived from the signal aspect.

Trainborne Equipment

The onboard equipment for the Chiltern Line Pilot is based on LZB trainborne equipment for urban traffic. The processor system itself is a 2 out of 2 configuration, where the two microprocessors work independently and compare their outputs by exchange of data. In normal operation, each of the computers outputs a life signal, which is individually supervised for correct sequence of pulses by a supervision board. Only if both supervision boards receive proper life signals, the emergency brake is released.
Outputs to the vehicle are by means of standard industrial non vital relays. For vital outputs, two relay contacts of different relays driven by different processors are used.
Parallel input data are fed to the processors by optocouplers. Where vital operation is required, two independent optocouplers read the information.

Operation of the SELCAB System

Three types of supervision are implemented:
• Train trip (when passing a signal at danger)
• Continuous supervision of speed
• Rollback supervision

The continuous supervision function can be split into two subtasks:
Supervision of a constant permissible speed which also is indicated to the driver. The system warns the driver and applies the service brake in case of no appropriate reaction.
Supervision of the braking action of the train. A number of continuous braking curves is calculated on board which each causes a defined reaction when being exceeded.

The first curve (indication curve) gives a brake announcement and initiates the indication of target data. The second curve (warning curve) generates an audible alarm. The third curve (service brake curve) automatically applies the service brake which can only be released after the permissible speed is reached. The last curve is only activated in case of a failure of the service brake (Emergency brake curve).
This brake can only be released when the vehicle has come to a complete stop.
All curves except the emergency brake curve are oriented at the position of the signal. In case of an emergency brake application the vehicle is stopped within the overlap. This curve points to the end of the overlap.
If there is more than one target transmitted, a minimum selection for the actual position of the vehicle is made. Up to four targets can be processed at a time.

Glasgow CTS Contactless Trainstop

This description was reproduced with permission from an article in IRSE News 109, November 2005 by Ed. Gerrard and David Hughes.

CTS Trackside Equipment Architecture and Operation

The track beacon comprises two polarized permanent magnets and two coils, a signal coil and a feedback coil and these coils are mutually coupled.
Housed in the local Station Switchroom there is a transmitter, which is switched on by a call for the corresponding signal to be switched to Green (DR). This sends a 25.6 kHz signal to the beacon signal coil to disarm the beacon. To prove that the beacon has been disarmed a beacon feedback coil picks up the signal and feeds it back to a detector which on receipt of this feedback signal energises a relay (CTSPR).
This interfaces with conventional train stop circuitry using the VPR and VCR relays. These proving relays then allow the trackside Signal lamp to be switched to Green if both DR and CTSPR/VPR are energised. If the Signal is at Red then there is no transmitter signal and the Beacon is armed. In this scenario only the permanent polarised magnets are present and both the DR and CTSPR/VPR are de-energised. Contacts of the DR and CTSPR relays are monitored and if they are out of correspondence for whatever reason, a train stop alarm is given through the Supervisory System in the central control room. This alarm can then be reported to the maintenance department so that action may be taken.
As per the existing mechanical train stop system in normal operation, the disarming of a beacon at Station one, due to the signal being at Green, prevents the Station two in rear train stop from disarming and clearing that station's protecting signal until the train at Station one is clear of Station one's Overlap. In the event of a train passing through Station one train stop, which does not eventually re-arm, then the protecting signal in rear at Station two will not clear, even though the leading train has cleared the forward protected section. In this situation a train stop failure alarm for Station one is given in the central control room.

CTS Train Carried Equipment Architecture and Operation

The train¬carried sensor consists of two saturable coils and an aerial. The two saturable coils are connected to two separate oscillator circuits, which are ultimately connected to a logic array decoder, and a Signal aerial whose signal is again
connected into a logic array decoder. When the train passes over a beacon with only the permanent polarized magnets and no frequency signal, in the correct direction, the saturable coils kill the two oscillators and an Emergency Brake Application is actuated through the logic array, interface card and brake solenoid.
When the train passes over a beacon that is energised by the 25.6 kHz signal, this frequency signal is detected first and suppresses the train trip called for by the saturable coils passing over the permanent magnets. A short comfort beep is given to the driver to indicate that the CTS system has just passed over a disarmed beacon and is operating normally. In the event that the Sensor sees only a 25.6 kHz signal and not the permanent magnets, then an audible alarm is given to the Driver indicating an error. The driver will then be required to obey restrictive operational rules in this situation.
In passenger service the normal mode of driving is Automatic (ATO) with the possibility of the train being driven in Unrestricted Manual mode. In the event of a Trip at a red signal that produces an emergency brake application the operator is required to reset the train-carried CTS unit. Upon resetting the CTS unit the driving mode is automatically forced to 'Restricted Manual' operation (25 km/h speed limit). This speed limit remains in force until the train passes a valid disarmed (green) beacon, which will usually be at the next station. When the on-board CTS unit receives the signal from this valid beacon, then the speed restriction is automatically removed and normal driving is re-established.
The main CTS train-carried control equipment is installed under one of the lockable passenger seats. The driver interface and controls (reset etc.) for the CTS system are installed within the cab area in a locked enclosure for access when required.
The maximum speed on Glasgow Subway is only 60 km/h though the specification for the train-carried equipment is quoted as able to trip vehicles traveling up to 250 km/h.