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ABSRACT The CBTC, communications-based train control (CBTC) system under development, is the key element of the integrated automatic train control network in the Metros. It accomplishes vital functions providing safety of traffic movement. The CBTC system performs automatic train speed regulation utilizing a radio channel for train-to-wayside data communications. The integrated network is combined to perform the safety critical Automatic Train Protection (ATP), as well as Automatic Train Supervision (ATS)

and Automatic Train Operation (ATO) functions. Each

function is provided by the

additional fail-safe hardware which is installed

along with the appropriate software

utilizing mutual data communications

architecture. This paper describes the configuration, design principles, operational algorithms and the number of technical characteristics of the CBTC including the radio communications subsystem. The design and implementation rules are outlined and described in“Development of the communications-based train control system for Moscow Metro”, Minin V.A, Shishliakov, V.A Holyoak, IEEE 2007

TABLE OF CONTENTS CHAPTER

PAGE NO:

Figure list

i

Table list

ii

01. Introduction

1

02. Train control

2

2.1 Train Protection

2

2.2 Train Operation

2

2.3 Train Supervision

3

03. Train control block diagram

3

04. Existing systems vs CBTC

4

05. Signaling systems

4

5.1 block

5

5.2 train detection

7

5.3 Visual signaling

9

06. Interlocking

13

07. CBTC basic functional architecture

14

08. CBTC architecture

16

09. Working of CBTC

20

10. Communications channel architecture

22

11. Communications standards

23

12. Advantages & Disadvantages

23

13. Conclusion

24

13. References

25

FIGURE LIST FIGURE

NAME

PAGE NO:

FIG. 1

Block diagram of train control

3

FIG. 2

Track divided into blocks

5

FIG. 3

Track with Block Unoccupied

7

FIG. 4

Track with Block occupied

8

FIG. 5

Track with counter

9

FIG. 6

Wayside signaling

10

FIG. 7

Cab signaling

11

FIG. 8

Cab signaling speed codes

11

FIG. 9

Train born equipments for cab

13

FIG. 10

Example for interlocking

14

FIG. 11

CBTC basic functional architecture

15

FIG. 12

CBTC architecture

16

FIG. 13

Train borne equipments

19

FIG. 14

Working of CBTC

20

FIG. 15

Communications channel

22

i

TABLE LIST

TABLE

NAME

PAGE NO:

TABLE 1

Wayside to train message format

20

TABLE 2

Train to wayside message format

21

ii

1. INTRODUCTION In today’s railway industry, there are many different types of train control systems. The principal intent of a train control system is to prevent collisions when trains are traveling on the same track, either in the same direction (trains following one another) or in the opposite direction (two trains moving toward each other). These systems also permit safe movement of trains as they cross from one track to another. Early train control systems were very simplistic in architecture. As train technology and operation evolved over time, these control systems grew to have more and more complex architectures. The latest architecture is known as CBTC. As will be discussed, CBTC uses bidirectional radio frequency (RF) data communication between the trains and control locations distributed along the tracks (wayside).

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2. TRAIN CONTROL Train control is the process by which the movement of rail rapid transit vehicles is regulated for the purposes of safety and efficiency. The process is carried out by a combination of elements, some machine located on the train, along the track, in stations, and at remote central facilities. These elements interact to form a command and control system with three major functions: 1. Train Protection 2. Train Operation 3. Train Supervision

2.1 Train Protection Train protection is a family of functions whose purpose is to assure the safety of train movement by preventing collisions and derailments. The functions that make up train protection are: 1. Train detection 2. Train separation 3. Route interlocking 4. Over speed protection 5. Train and track surveillance

2.2 Train Operation Train operation consists of those functions necessary to move the train and to stop it at stations. Train operation involves the following 1. Train starting 2. Train speed regulation 3. Train stopping

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2.3 Train Supervision Train supervision involves monitoring the movement of individual trains in relation to schedule and route assignments and overseeing the general disposition of vehicles and flow of traffic for the system as a whole. The train supervision system may thus be thought of as making strategic decisions which the train operation system carries out actually, In addition, train supervision includes certain information processing and recording activities not directly concerned with train safety and movement but necessary to the general scheme of operations. Train supervision functions are •

Schedule design and implementation

•

Route assignment

•

Performance monitoring

3. TRAIN CONTROL BLOCK DIAGRAM

Fig no: 1 Block diagram of train control According to fig (1) train supervision do functions according to dispatcher commands. Train operation cause controls through regulators. In between them train protection occurs.

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4. EXISTING SYSTEMS Vs CBTC Existing system can do automated train protection, Manual train operation, and Manual train supervision. But CBTC can handle fully automated train protection, automated train operation, automated train supervision. So ATS (automatic train supervision) and ATC (automatic train control) systems rely on the relay of information through audio-frequency (AF) current to transmit ATS or ATC related information along the track circuit. This approach has some technical limitations. First, the location of trains can only be determined to the resolution of the track circuits. If any part of a track circuit is occupied, that entire track circuit must be assumed as occupied. The track circuit’s length can be made shorter, but adding additional track circuits requires additional wayside hardware. This imposes additional costs, causing a practical and economical limit to the number of track circuits that a railroad can install. Second, the information that can be provided to a train through a track circuit is limited to a small number of wayside signal aspects or speed data.

5. SIGNALING SYSTEMS Signaling is one of the most important parts of the many which make up a railway system. Train movement safety depends on it and the control and management of trains depends on them. Over the years many signaling and train control systems have been evolved so that today a highly technical and complex industry has developed. Major function of signaling systems are •

Give visual display of track conditions

•

Operate devices according to these signals

4

Main parts of any signaling systems are,

5.1 Block Railways are provided with signaling primarily to ensure that there is always enough space between trains to allow one to stop before it hits the one in front. This is achieved by dividing each track into sections or "blocks". Each block is protected by a signal placed at its entrance. If the block is occupied by a train, the signal will display a red "aspect" as we call it, to tell the train to stop. If the section is clear, the signal can show a green or "precede" aspect. The simplified fig (2) shows the basic principle of the block. The block occupied by Train 1 is protected by the red signal at the entrance to the block. The block behind are clear of trains and a green signal will allow Train 2 to enter this block. This enforces the basic rule or railway signaling that says only one train is allowed onto one block at any one time.

Fig no: 2 Track divided into blocks There are two types of blocks, they are •

Fixed block: block has fixed length.

•

Moving block: computers calculate block distance so can increase track capacity.

5.1.1 Fixed Block Most blocks are "fixed", i.e. they include the section of track between two fixed points. Blocks usually start and end at selected stations. The lengths of blocks are designed to allow trains to operate as frequently as necessary. A lightly-used line

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might have blocks many kilometers long, but a busy commuter line might have blocks a few hundred meters long. A train is not permitted to enter a block until a signal indicates that the train may proceed, a dispatcher or signalman instructs the driver accordingly, or the driver takes possession of the appropriate token. In most cases, a train cannot enter the block until not only the block itself is clear of trains, but there is also an empty section beyond the end of the block for at least the distance required to stop the train. In signaling-based systems with closely-space signals, this overlap could be as far as the signal following the one at the end of the section, effectively enforcing a space between trains of two blocks. When calculating the size of the blocks, and therefore the spacing between the signals, the following have to be taken into account •

Line speed (the maximum permitted speed each train)

•

Gradient (to compensate for longer or shorter braking distances)

•

The braking characteristics of trains on that line

•

Sighting (how far ahead a driver can see a signal)

•

Reaction time (of the driver)

5.1.2 Moving Block One disadvantage of having fixed blocks is that the faster trains are allowed to run, the longer the stopping distance, and therefore the longer the blocks need to be, thus decreasing the line's capacity. Under a moving block system, computers calculate a 'safe zone' around each moving train that no other train is allowed to enter. The system depends on knowledge of the precise location and speed and direction of each train, which is determined by a combination of several sensors. With a moving block, line side signals are unnecessary, and instructions are passed directly to the trains. This has the advantage of increasing track capacity by allowing trains to run closer together while maintaining the required safety margins.

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5.2 Train Detection For train detection two methods are used they are, 1. Track circuits 2. Axle counters

5.2.1 Track circuits Nowadays for signaling purposes, trains are monitored automatically by means of "track circuits". Low voltage currents applied to the rails cause the signal, via a series of relays (originally) or electronics (more recently) to show a "proceed" aspect. The current flow will be interrupted by the presence of the wheels of a train. Such interruption will cause the signal protecting that section to show a "stop" command. Any other cause of current interruption will also cause a "stop" signal to show. Such a system means that a failure gives a red aspect – a stop signal. The system is sometimes referred to as "fail safe" or "vital". A "proceed" signal will only be displayed if the current does flow. Most main lines with moderate or heavy traffic are equipped with color light signals operated automatically or semi-automatically by track circuits.

5.2.1 .1 Track Circuit - Block Unoccupied Figure (3) shows how the track circuit is applied to a section or block of track. A low voltage from a battery is applied to one of the running rails in the block and returned via the other. A relay at the entrance to the section detects the voltage and energizes to connect a separate supply to the green lamp of the signal.

Fig no: 3 Track with Block Unoccupied 7

5.2.1.2 Track Circuit - Block Occupied When a train enters the block (fig 4), the leading wheel set short circuits the current, which causes the relay to de-energies and drop the contact so that the signal lamp supply circuit now activates the red signal lamp. The system is "fail-safe", or "vital" as it is sometimes called, because any break in the circuit will cause a danger signal to be displayed. A block section is normally separated electrically from its neighboring sections by insulated joints in the rails. However, more recent installations use electronics to allow joint less track circuits. Also, some areas have additional circuits which allow the signals to be manually held at red from a signal box or control centre, even if the section is clear. These are known as semi-automatic signals. Even more complexity is required at junctions.

Fig no: 4 Track with Block occupied

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5.2.2 Axle Counters An alternative method of determining the occupied status of a block is using devices located at its beginning and end that count the number of axles entering and leaving. If the same number leaves the block as enter it, the block is assumed to be clear. Figure (5) shows block diagram of axial counting technology.

Fig no: 5 Track with counter

5.3 Visual signaling For giving visual identification of ahead track two signaling systems are mainly used they are •

WAYSIDE SIGNALLING

•

CAB SIGNALLING

5.3.1 Wayside signaling When this was developed for track circuited signaling, the caution signal was provided a block further back from the stop signal. Each signal would now show a red, yellow or green aspect - a multi-aspect signal. When this was developed for track circuited signaling, the caution signal was provided a block further back from the stop signal. Each signal would now show a red, yellow or green aspect a multi-aspect signal. Fig (6) shows a line with 3-aspect signals. The block occupied by Train 1 is protected by the red signal at the entrance to the block. The block behind is clear of trains but a yellow signal provides advanced warning of the red aspect ahead. This block provides the safe braking distance for Train 2. The next block in rear is also clear of trains and shows a green signal. The driver of Train 2 sees the green signal 9

and knows he has at least two clear blocks ahead of him and can maintain the maximum allowed speed over this line until he sees the yellow.

Fig no: 6 Wayside signaling

5.3.2 Cab signaling Cab signaling is a system that communicates track status information to the train cab (driving position), where the engineer or driver can see the information. The simplest systems display the trackside signal aspect, while more sophisticated systems also display allowable speed and dynamic information about the track ahead. In modern systems, a train protection system is usually overlaid on top of the cab signaling system to warn the driver of dangerous conditions, and to automatically apply the brakes and bring the train to a stop if the driver ignores the dangerous condition. Cab signaling systems range from simple coded track circuits to transponders that communicate with the cab and communication based train control systems. ATP signaling codes contained in the track circuits are transmitted to the train. They are detected by pick-up antennae (usually two) mounted on the leading end of the train under the driving cab. This data is passed to an on-board decoding and safety processor.

The permitted speed is checked against the actual speed and, if the

permitted speed is exceeded, a brake application is initiated. In the more modern systems, distance-to-go data will be transmitted to the train as well. The data is also sent to a display in the cab which allows the driver of a manually driven train to respond and drive the train within the permitted speed range.

10

At the trackside, the signal aspects of the sections ahead are monitored and passed to the code generator for each block. The code generator sends the appropriate codes to the track circuit. The code is detected by the antennae on the train and passed to the on-board computer. The computer will check the actual speed of the train with the speed required by the code and will cause a brake application if the train speed is too high. Figure (7) shows cab signaling system.

Fig no: 7 Cab signaling A train on a line with a modern version of ATP (automatic train protection) needs two pieces of information about the state of the line ahead, what speed it can do in this block and what speed must it is doing by the time it enters the next block. This speed data is picked up by antennae on the train. The data is coded by the electronic equipment controlling the track circuitry and transmitted from the rails. The code data consists of two parts, the authorized speed code for this block and the target speed code for the next block. Fig (8) shows how this works.

Fig no: 8 cab signaling speed codes 11

In this example (left), a train in Block A5 approaching Signal A4 will receive a 40 over 40 codes to indicate a permitted speed of 40 km/h in this block and a target speed of 40 km/h for the next. This is the normal speed data. However, when it enters Block A4, the code will change to 25 because the target speed must be 25 km/h when the train enters the next Block A3. When the train enters Block A3, the code changes again to 25 because the next block (A2) are the overlap block and are forbidden territory, so the speed must be zero by the time train reaches the end of Block A3. If the train attempts to enter Block A2, the on-board equipment will detect the zero speed code 0 and will cause an emergency brake application. As mentioned above, Block A2 is acting as the overlap or safe braking distance behind the train occupying Block A1.Transferring the display of information from the wayside to the cab involves an alternate type of track circuit technology. To operate cab signals, the current passing through the track circuit (usually ac. is not steady, as for conventional wayside signals, but is pulsed (turned on and off) at several different repetition rates in response to track occupancy. Each pulse rate is a code to indicate allowable train speed. This pulsed dc Energy is passed through the rails, picked up inductively by receiver (antenna) on the train, and decoded to retrieve speed command information; this information is used to actuate the appropriate cab signal display. Because the train is continuously receiving pulses of energy, a change in the pulse rate of the coded track circuits indicating a change of conditions ahead of the train is instantaneously received by car borne equipment and displayed by cab signals regardless of where the train happens to be within a block. Fig (9) shows train borne equipments for cab signaling.

12

Fig no: 9 Train born equipments for cab signaling Main advantages of cab signaling are, •

Clear display of track conditions

•

Can used with CBTC

•

Can used for high speed railway

•

Free from fog, rain, and snow

6. INTERLOCKING An interlocking is an arrangement of signals and signal appliances so interconnected that functions must succeed each other in a predetermined sequence, thus permitting safe train movements along a selected route without collision. Fig (10) shows such a situation, in which train1 coming from track 1 and train2 coming from track 2. Train 1 need to go to track 2. If priority given to train 1,

13

interlocking devices connect track 1 with track 2 and associated signals should operate to avoid collision.

Fig no: 10 examples for interlocking There are many types of Interlocking •

Mechanical interlocking

•

Electro-mechanical interlocking

•

Electronic interlocking (used in CBTC)

7. CBTC BASIC FUNCTIONAL ARCHITECTURE CBTC systems are complex systems made up of distributed physical, but closely coupled, functional subsystems. Their successful operation requires a well orchestrated set of interactions. Understanding the basic CBTC Architecture, CBTC functional requirements, and modes of operations assist in understanding a CBTC system. All such CBTC systems are derivations of a single basic functional architecture, with specific enhancements and modifications to both functions and modes of operations to support the unique requirements and operational needs of the individual railroad the system. The basic functional architecture, illustrated in Figure (11) consists of three major functional subsystems, wayside, mobile, and dispatch/control. The wayside subsystem consists of elements such as highway grade crossing signals, switches and interlocks or maintenance of way workers. The mobile subsystem consists of

14

locomotives or other on rail equipment, with their onboard computer and location systems. The dispatch/control unit is the main section that runs the railroad. Each major functional subsystem consists of a collection of physical components implemented using various databases, data communications systems, and information processing equipment.

Fig no: 11 CBTC basic functional architecture Link between wayside units –mobile units and central units-wayside units are RF communication.

Link

between

central units-mobile

communication.

15

units

are

fiber

optic

8. CBTC BASIC FUNCTIONAL ARCHITECTURE Figure (12) shows CBTC basic functional architecture.

Fig no: 12 CBTC basic functional architecture It consist of basic three sections they are, •

Wayside units

•

Mobile units

•

Control centers

Each section consist of following parts, 1. Mobile unit Interrogator antenna (AI) Antenna (A) Train borne equipments 2. Wayside units RF transmitting cables Optical fiber cables Inter locking and other control equipments

16

2. Control centers Base data radio (BDR) User terminal (UT) System controller (SC) Communication controller (CC) Communication equipments (CE)

The system hardware is installed at the System Control Center (SCC) location, at the stations, On board the trains and along the track. The track coordinate is established by the programmable transponder tag numbers. Transponder tags (T) are spaced approximately 100 meters and installed on the ties. A distributed communications system leaky feeder cable is installed in each tunnel along the track in order to provide reliable radio coverage. As the vehicle moves along the track the train borne equipment retrieves the track coordinate from the tags through interrogator antenna (AI) and transmits information to the system control center via the radio channel. The base data radio (BDR) at the control center receives the transmitted data and transfers it to the redundant local area network (LAN) via the communications controller (CC). The information concerning switches, signals and track circuits at the stations located within the constraints of the system is also supplied to the LAN by the CC. Station interfaces obtain this data from the existing interlocking equipment and transfer it to the CC via fiber- optic communications links. These links are made up of the main data circuits (DCl), redundant data circuits (DC2), and redundant communications equipment (CE). The system controller (SC) responsible for the proper system algorithm implementation is connected to the LAN.

Personal

computers, functioning as a user terminal (UT) and a technical terminal (TT), are also linked to the LAN. In order to comply with the safety standards, the station interface and the communications controller are built upon two off-the-shelf processor assemblies. System controller (SC) accommodates three processor assemblies to provide for the 17

cross checked redundant architecture. The processor units perform data processing independently to verify correspondence on any output. If there is a conflict between the channels, the vital blocking unit (BU) halts the failed equipment and activates the backup devices. The SCC can be easily reconfigured to perform ATS and ATO functions by linking controllers of the dispatcher terminals, the centralized traffic control board and the ATO to LAN. By that time the SI must carry supplementary boards for the centralized electrical control. The software needs to be modified to provide for extra data streams in the communications system and LAN. The structure

and

components of the

system controller’s

permanent

databases were determined according to the detailed knowledge of the system’s physical constraints including grades, speed restriction, curvatures, switching and interlocking location, station data, etc.The databases for the track sections contain section

numbers,

track coordinates with

the appropriate parameters, possible

direction data (forward, reverse), and data for transition to the subsequent database including cases of varied switch positions. The track databases also include the track section descriptive parameter (tunnel, above the ground, spur), gradient of the track, restricted speed at the point of the planned obstacle, and the ID of the base radio servicing the referenced track section.

8.1 Train borne Equipments It consist of following components •

On board computers (OBC)

•

Mobile data radio (MDR)

•

Control unit(CU)

Figure (13) presents the train borne equipment configuration block diagram. The information packet is received by the mobile radio (MDR) in the train within the CBTC territory. This packet will be stored in the on-board computer (OBC) memory only if the train number in the OBC’s database corresponds to the message

18

address field N. If the train is just entering the system’s territory, the packet will be stored in the OBC upon the agreement between the entrance point numbers received by the interrogator I and the pertinent message field Xe. Dependent on

the

acceptance of the data packet, the OBC determines safe speeds and builds, if applicable, the braking command. The OBC transfers the information message to the SCC via the MDR.VB, VC, RB, C are commands, these will explain later.

Fig no: 13 train borne equipments

9. WORKING OF CBTC

Fig no: 14 working of CBTC When train enter the block it receive message from wayside about obstacle speed, distance etc. according to these information it calculate safe braking distance and other control parameters. Message formats given below. 19

9.1 Wayside to Train Message Format

Table no: 1Wayside to train message format N

: train number

Nc

: cab number

Xe

: token number

Nr

: BDR identification number

L

: train length

Vob

: obstacle speed

ES

: emergency stop

9.2 Train to Wayside Message Format

Table no: 2 Train to wayside message format N

: train number

Nc

: cab number

Xi

: tag number

Xj

: distance from tag

Lt

: train length

Vt

: train speed

M

: mode of operation (acceleration, coasting and free running)

BR

: brake state

20

According to the information from the SC and breaking curves database stored in the OBC’s memory, the OBC determines two permissible speeds Vps and Vp. The first one Vps is established for the full service breaking mode. If the train’s actual speed exceeds Vps he OBC issues the “RB” command (run break) to the on-board control unit (CU) to stop acceleration and initiate the application of the electric brake. The failure in the break control circuit is reported to the OBC by the “VC” command (vital check) from the train’s check-over circuit CC. In response, the OBC issues the command “VB”- vital break - to de-energize the electro pneumatic relay (EPR). This action results in the emergency breaking using safe pneumatic break. The commands “VC” and “VB”, and also some of the commands that are entered from the train’s Control panel (CP) are vital and are transmitted via the vital circuits designed according to safety standards.

The command “RB” and other

control signals “C” are sent via non-vital circuits. Vital commands are never transmitted over the radio link. The speed Vp is determined for the service break mode and is less than Vps. This mode is more comfortable for the passengers and the train itself. Vp is displayed on the train display (D), as well as the actual speed Vt and the distance L between the train and the obstacle. The train engineer can manually reduce the train speed to reach the Vp value. If the OBC is outfitted with ATO software, the train speed can be reduced automatically using RBC circuits. Upon detection of the MDR, interrogator I or an axle generator failure the OBC will establish communications with the OBC in the trail car.

10. COMMUNICATIONS CHANNEL ARCHITETURE

The reliability of the CBTC system depends a great deal on the architecture and characteristics of data communication channels. The data communications architecture is effectively combined by the train-to-wayside communications system and the wayside communications network. According to one of the design approaches, the train-to-wayside communications channel will employ the Andrew

21

Model 2400 data radios and RADIAX® leaky feeder cable. This system will provide contactless two-way data communications between CBTC control center SCC and the train borne OBC. The communications system components form the following topology illustrated in Figure.

Fig no: 15 Communications channel BDR is controlled by the CBTC system's communications controller (CC) and interfaces directly with the CC via a standard EIA-530 serial interface. Similarly, the MDR interfaces to the train borne OBC via an EIA- 530 interface. MDR is very similar to the BDR except that it is ruggedized to operate in severe environments. The MDR has complementary transmit and receive frequencies to the BDR. Actual locations of the BDR are based on the physical architecture of the train control system. The architecture is usually composed of a number of control regions. These control regions are typically defined by system topological characteristics like the configuration of tracks, stations, spurs, etc. Each region would include the BDR using a region specific code to communicate with the trains in that region. (This technique is often referred as Code Division Multiple Access CDMA).

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11. COMMUNICATIONS STANDARDS

Protocol using

: high data link connection (HDLC)

Type of link

: full duplex type

Digital modulation Methods

: BFSK

Channel access method

: CDMA

12. ADVANTAGES AND DISADVANTAGES Main advantages and disadvantages of CBTC are 1. Increased rail capacity through closer train operation 2. Improved efficiency and flexibility of the rail network 3. Improved service reliability 4. Increased safety 5. Reduced operation and maintenance cost for the trackside infrastructure Main disadvantages are 1. amount of data can transmit through channels is limited 2. Initial cost high 3. Need experienced workers 4. Complex system

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13. CONCLUSION The described CBTC system is designed to ensure enhanced safety of traffic movement in conjunction with headway minimization. The

system can be

implemented either as an independent replacement or gradually as an overlay of current systems, without disturbing the operation of the trains outfitted with the existing hardware as well as with new technology equipment. The train-towayside communications channel based on spread spectrum communications techniques and leaky feeder cable technology offers many advantages in electrically noisy

metro

environment.

an

This approach provides maximized

interference immunity and, therefore, high reliability and availability of the CBTC system.

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13. REFERENCES

1. Hongli Zhao, Tianhua Xu, ‘Towards modeling and evaluation of availability of communication based train control (CBTC) system’, IEEE 2009 2. Mirtchev, ‘Automatic restart for communication based train control systems’, IEEE 2005 3. Morar s, ‘Evolution of Communication Based Train Control worldwide’ , ,IEEE 2010 4. Rober d Pascoe and Thomas, ‘What is communication-based train control?’, IEEE 2009 5. Site : http://www.railway-technical.com

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