Summer Training Report
Vaibhav Tiwari nd
B. Tech 2 Year SRM University, SRM Nagar Kattankulathur – 603203, Kancheepuram District, Chennai Tamilnadu
This is to certify that
(1041210250) student of
2011-2015 Batch of Electronics & Communication Branch in 2nd Year of
SRM University, Kattankulathur , Chennai
has
successfully completed completed his industrial training at Delhi Metro Rail Corporation Ltd., Yamuna bank depot, New Delhi for six weeks from 16th June to 15st July 2014. He has completed the whole training as per the training report submitted by him.
Training In-charge Delhi metro rail corporation Ltd. Yamuna bank depot, New Delhi
This is to certify that
(1041210250) student of
2011-2015 Batch of Electronics & Communication Branch in 2nd Year of
SRM University, Kattankulathur , Chennai
has
successfully completed completed his industrial training at Delhi Metro Rail Corporation Ltd., Yamuna bank depot, New Delhi for six weeks from 16th June to 15st July 2014. He has completed the whole training as per the training report submitted by him.
Training In-charge Delhi metro rail corporation Ltd. Yamuna bank depot, New Delhi
TABLE OF CONTENT:-
1. Acknowledgement 2. About the company 3. SYSTEM OVERVIEW OF METRO
Braking system
Traction Power supply
C-VIS
HSVCB
SCMS
Energy storage
SCADA
Signaling system
Rolling stock
4. OB communication overview
Train radio system TCMS CCTV ATO/ATP
Acknowledgement It’s a great pleasure to present this report of summer training in Delhi Metro
Rail Corporation (A Joint Venture of Govt. Of India and Govt. Of Delhi) in
partial fulfillment of B.Tech Programmed under
At the outset, I would like to express my immense gratitude to my training guide, Mr. Amit Giri , guiding me right from the inception till the successful completion of the training. I am falling short of words for expressing my feelings of gratitude towards him for extending their valuable guidance, through critical reviews of project and the report and above all the moral support he had provided me with all stages of this training.
ABOUT THE COMPANY •
Delhi Metro Rail Corporation (DMRC) was established by the Government of India and the Government of Delhi in March 1995 to build a metro system in the capital.
•
The metro network consists five lines with total length of 125.67kms.
•
The metro has 80 stations and 28 are underground.
•
Construction work in progress for the phase-IV.
Finances and Funding From government of India and government of Delhi contribute equal shares, trough soft loan from Japan bank for international cooperation.
Revenue and Profits Revenue from advertisements and property development, leasing out trains stations for film shoots.
Security •
Central industrial security force (CISF)
•
Closed circuit cameras
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Dog squads
•
Emergency communication b/w passengers and driver.
DMRC NETWORK:-
SYSTEM OVERVIEW OF METRO:-
Braking system in Delhi Metro Train •
Its normal braking actuated by train operator using TBC during normal train operation.
•
It’s a mixture of regenerative braking and electro pneumatic friction
brake
Traction Power supply to Delhi Metro Train •
Power is supplied by 25 kv, 50 Hz ac through overhead catenary.
•
1. (C- VIS)- Cubicle type Vacuum Insulated Switchgear 2. (HSVCB)- High Speed Vacuum Circuit Breaker 3. (B-CHOP)- Energy Storage for Traction Power Supply System 4. (SCMS)- Stray Current Monitoring System •
SCADA system for power supply and network equipment surveillance.
C-VIS (Cubicle type Vacuum Insulated Switchgear) •
25 kV vacuum insulated switchgear in order to eliminate the risk of greenhouse gas emission, to meet customer requirement such as compact design and low maintenance.
•
1. Dual Contact design (High reliability of interrupt and disconnect) 2. SF6 gas free -Vacuum Insulation 3. Compact design 4. Grease free
HSVCB (High speed vacuum circuit breaker) •
Hitachi contributes the electric railroad system demanded to the safer service through HSVCB which is unique to us.
•
1. Low noise 2. No arc emission 3. Very short time interruption 4. Low maintenance
Stray Current Monitoring System (SCMS) This system provides evaluation of the stray current conditions of the track, which facilitates early detection of insulation deficiencies and allows necessary measures to be taken to prevent potential damages caused by stray current corrosion
B-Chop (Energy Storage for Traction Power Supply System)
SCADA (Supervisory control $ data acquisition) •
SCADA is a software system which is in charge of surveillance and data collection by personal computers.
Signaling system of Delhi metro Signaling used on high density metro (or subway) routes is based on the same principles as main line signaling. The line is divided into blocks and each block is protected by a signal but, for metros, the blocks are shorter so that the number of trains using the line can be increased. They are also usually provided with some sort of automatic supervision to prevent a train passing a stop signal. •
1. Control all operation from acceleration to stopping. 2. Realize driverless operation. •
1. Used for making high speed operation. 2. It detect train position and transmit signal to control unit.
Figure 1: Diagram showing simple Metro-style two-aspect signaling.
Originally, metro signaling was based on the simple 2-aspect (red/green) system as shown above. Speeds are not high, so three-aspect signals were not necessary and yellow signals were only put in as repeaters where sighting was restricted. Many metro routes are in tunnels and it has long been the practice of some operators to provide a form of enforcement of signal observation by installing
additional equipment. This became known as automatic train protection (ATP). It can be either mechanical or electronic. The older, mechanical version is the train stop; the latter, electronic version depends on the manufacturer. The train stop consists of a steel arm mounted alongside the track and which is linked to the signal. If the signal shows a green or proceeds aspect, the train stop is lowered and the train can pass freely. If the signal is red the train stop is raised and, if the train attempts to pass it, the arm strikes a "trip cock" on the train, applying the brakes and preventing motoring. Electronic ATP involves track to train transmission of signal aspects and (sometimes) their associated speed limits. On-board equipment will check the train's actual speed against the allowed speed and will slow or stop the train if any section is entered at more than the allowed speed.
If a line is equipped with a simple ATP which automatically stops a train if it passes a red signal, it will not prevent a collision with a train in front if this train is standing immediately beyond the signal.
Figure 2: Diagram showing the need for a safe braking distance beyond a stop signal.
There must be room for the train to brake to a stop - see the diagram above. This is known as a "safe braking distance" and space is provided beyond each signal to accommodate it. In reality, the signal is placed in rear of the entrance
to the block and the distance between it and the block is called the "overlap". Signal overlaps are calculated to allow for the safe braking distance of the trains using this route. Of course, lengths vary according to the site; gr adient, maximum train speed and train brake capacity are all used in the calculation.
Figure 3: Diagram showing a signal provided with an overlap. The overlap in this example is calculated from the emergency braking distance required by the train at that location.
This diagram (Figure 3) shows the arrangement of signals on a metro where signals are equipped with train stops (a form of mechanical ATP) and each signal has an overlap whose length is calculated on the safe braking distance for that location. Signals are placed a safe braking distance in rear of the entrances to blocks. Signal A2 shows the condition of Block A2, which is occupied by Train 1. If Train 2 was to overrun Signal A2, the raised train stop (shown here as a "T" at the base of the signal) would trip its emergency brake and bring it to a stand within the overlap of Signal A2.
Figure 4: Diagram showing a train standing in the signal overlap.
Nothing in the railway business is as simple as it seems and so it is with overlaps. A line which uses overlaps and has close headways could have a situation as shown above where the train in the overlap of Signal A121 has a green signal showing behind it. Although it is protected by Signal A123 showing red, the driver of Train 2 may see the green signal A121 behind Train 1 and could "read through" or be confused under the "stop and proceed" rule.
Figure 5: Diagram of the track circuited overlap, sometimes known as a "replacing track circuit".
So, where there is a possibility of a green signal being visible behind a train, overlaps are track circuited as shown in Fig. 5. Although there is no train occupying the block protected by Signal A121, the signal is showing a red aspect because the train is occupying the overlap track circuit or "replacing" track circuit, as it is sometimes called.
This will give rise to two red signals showing behind a train whilst the train is in the overlap. The block now has two track circuits, the "Berth" track and the "replacing" track.
Figure 6: Schematic showing the principle of the Absolute Block system. Signal A127 is clear because two blocks in advance of it is clear. A125 shows a danger aspect because one of the blocks ahead of it is occupied by a train.
Many railways use an "Absolute Block" system, where a vacant block is always maintained behind a train in order to ensure there is enough room for the following train to be stopped if it passes the first stop (red) signal. In Figure 6, in order for Signal A125 to show a proceed aspect (green), the two blocks ahead of it must be clear, with Train 1 completely inside the block protected by Signal A121.
ROLLING STOCK The first wave of rolling stock was manufactured by a consortium comprising Hyundai Rotem, Mitsubishi Corporation and Mitsubishi Electric Corporation. Initial sets were built by ROTEM in South Korea, with later examples completed in India by public sector undertaking Bharat Earth Movers Limited (BEML). BEML is also responsible for the manufacturing coaches under technology transfer agreement. The air-conditioned trains consist of four 3.2m-wide, stainless steel, lightweight, although eight is possible. The trains have automatic doors, secondary air suspension and brakes controlled by microprocessor. Delhi Metro has a fleet of 280 coaches, which DMRC runs as 70 trains every day. Each train can accommodate about 1,500 people, 240 seated. Maximum speed is 80km/h (50mph), with a 20-second dwell time at stations. Train depots are located at Khyber Pass, Najafgarh, Shastri Park and Yamuna Bank. In May 2011, BEML received a contract worth Rs9.2bn ($205m) from DMRC to supply 136 intermediate metro cars. The delivery is expected to be completed by December 2013. In March 2008 Bombardier Transportation announced an 87m ($137m) contract for 84 MOVIA metro cars, a follow-on to an order for 340 placed in July 2007. The new vehicles are being deployed as part of the Phase II expansion. In September 2011, Bombardier received a $120m order for 76 additional MOVIA metro cars. This was a follow-on contract to an order placed for 114 vehicles in the middle of 2010. Deliveries under the new order are expected to be completed between the third quarter of 2012 and early 2013. DMRC received the first MOVIA metro car from Germany in February 2009. The first 36 vehicles will be manufactured in Gorlitz, Germany, and the remaining 388 cars will be built at Bombardier's Indian manufacturing facility in Savli, South Gujarat.
broad gauge train, supplied by broad gauge train, supplied by The Metro uses rolling stock of two different gauges. Phase I lines use 1,676 mm (5.499 ft) broad gauge rolling stock, while three Phase II lines use 1,435 mm (4.708 ft) standard gauge rolling stock. Trains are maintained at seven depots at Khyber Pass and Sultanpur for the Yellow Line, Mundka for the Green Line, Najafgarh and Yamuna Bank for the Blue Line, Shastri Park for the Red Line and Sarita Vihar for the Violet Line.
The broad gauge rolling stock is manufactured by two major suppliers. For the Phase I, the rolling stock was supplied by a consortium of companies comprising Hyundai Rotem, Mitsubishi Corporation, and MELCO. The coaches were initially built in South Korea by ROTEM,[116] then in Bangalore by BEML through a technology transfer arrangement. These trains consist of four 3.2metre (10 ft) wide stainless steel lightweight coaches with vestibules permitting movement throughout their length and can carry up to 1500 passengers, with 50 seated and 330 standing passengers per coach. The coaches are fully air conditioned, equipped with automatic doors, microprocessor-controlled brakes and secondary air suspension, and are capable of maintaining an average speed of 32 km/h (20 mph) over a distance of 1.1 km (0.68 mi). The system is extensible up to eight coaches, and platforms have been designed accordingly. The rolling stock for Phase II is being supplied by Bombardier Transportation, which has received an order for 614 cars worth approximately US$ 1100 million. While initial trains were made in Germany and Sweden, the remainder will be built at Bombardier's factory in Savli, near Vadodara These trains are a mix of four-car and six-car consists, capable of accommodating 1178 and 1792 commuters per train respectively. The coaches possess several improved features like Closed Circuit Television (CCTV) cameras with eighthour backup for added security, charging points in all coaches for cell phones
and laptops, improved air conditioning to provide a temperature of 25 degrees Celsius even in packed conditions and heaters for winter.
The standard gauge rolling stock is manufactured by BEML at its factory in Bangalore. The trains are four-car consists with a capacity of 1506 commuters per train, accommodating 50 seated and 292 standing passengers in each coach. These trains will have CCTV cameras in and outside the coaches, power supply connections inside coaches to charge mobiles and laptops, better humidity control, microprocessor-controlled disc brakes, and will be capable of maintaining an average speed of 34 km/h (21 mph) over a distance of 1.1 km (0.68 mi) position. This prevents any kick from the pipe as it is disengaged. Closing the angle cocks also has the effect of bleeding off the air trapped in the hose. The angle cock has a special bleed hole for this purpose.
OB COMMUNICATION The train radio system is the main link for non-safety critical vehicle communication. The system can handle both voice and data communication in order to – Allow operation control center (00C) to read status information from the vehicle.
Allow the driver to speak with OCC and/or depot. Allow OCC to perform remote operation of the vehicle PIS. Allow OCC to passively supervise cab activities, i.e. the current voice/sound of the active cab
DT car
hifT-car
19 Trainborne rack
1
Radio centre/ bead (RCF-1)
1
Train radio control panel (TRCP)
1
-
Speaker
1
_
Handset
1
Antenna
1
Fist microphone
1
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Train radio system units in driving cab The train driver will see five items, in the driving cab. that make up the train radio system:
19" sub-rack. located behind the co-driver's seat
Train radio control panel (1RCP) mounted on the left hand
sidewall of the driver.
Radio control head (RCN) mounted on the left hand sidewall of the driver.
Handset mounted on the console in front of the driver seat t o be used as default option for voice input.
Fist microphone mounted on the left hand sidewall of the driver to be used as backup option for voice input.
:The function of TCMS is to control and monitor on board systems and sub systems connected to the train communication network. The TCMS system incorporates unit and train level functionality for the different systems that has interlaces with the TCMS system. it is a distributed and modular system. The following functions/systems are supervised /controlled by TCMS :
Propulsion
Brakes Auxiliary electric system
Train operation control
Doors Passenger information system
ATP/ATO
Train radio Air supply
Carbody fittings
Interior Coupler HVAC Line voltage
Battery
Fire detection
CCTV
Units in TCMS
CCU-0 CC U-C MOBAD MIO-DX2 MIO-DX3 MIO-DX4 AX MCG Antenna Dual-
Central computing unit — operational Central corn utin unit — comfort Mode/Batter /Address unit Modular di ital in ut/out ut unit Modular di ital in ut/out ut unit Modular di ital in ut/out ut ur.: Analo ue in ut /out ut unit Mobile communication atewa
11M1
Human machine interface
1 1 3 2 1 1 1 1 1 1
1
1
1 1
1 1
TCMS software - uploader: Offers the user an interface for uploading or reading the information stored in the diagnostic system. It enables the maintenance staff to view and analyze the information uploaded from the on-board TDS system. It is a software tool for the maintenance personnel. Software is used to view analog and logical signals in real-time in a graphical environment, to analyze the system status, to analyze the operation-recording of signals, to enable test procedures through buttons and scripts. MTVD is a tool mainly for the maintenance personnel.
:
The main function of CCTV system is to record the events in the saloon area & Platform. Cameras are directly connected to the DVRs in the DT-car It, other cars cameras are connected to remote units. All images are streamed to the DVRs where they are stored. The DVRs and remote units are connected to the TCMS via IP backbone. The CCTV system via DVF-i will communicate with the TCMS via IP backbone. Live camera images can be viewed on monitors in both cabs. When the vehicle is activated, it performs a system start-up and supplies power to the CCTV system.
After the system start-up, tile video system starts recording images.
When there is no power, the CCTV system de-activates.
AUTOMATIC TRAIN PROTECTION (ATP)/AUTOMATIC TRAIN OPERATION (ATO):-
To drive trains between stations and slop them with high precision. To give consistent speed profile for at trains to improve both traffic regularity and Line capacity.
ATO
ATP is the safety system which ensures that trains remain a safe distance a part and have sufficient warning to allow them to stop without colliding with another train. ATO (Automatic Train Operation) is the non-safety part of train operation related to station stops and starts. The basic requirement of ATO is to tell the train approaching a station where to stop so that the complete train is in the platform. This is assuming that the ATP has confirmed that the line is clear.
The train approaches the station under clear signals so it can do a normal run in. When it reaches the first beacon - originally a looped cable, now usually a fixed transponder - a station brake command is received by the train. The on board computer calculates the braking curve to enable it to stop at the correct point and, as the train runs in towards the platform, the curve is updated a number of times (it varies from system to system) to ensure accuracy. Modern systems require less wayside checking because of the dynamic and more accurate on-board braking curve calculations. Now, modern installations can achieve ± 0.15 meters stopping accuracy - 14 times better.
ATO works well when the line is clear and station run-ins and run-outs are unimpeded by the train ahead. However, ATO has to be capable of adapting to congested conditions, so it has to be combined with ATP at stations when trains are closely following each other. Metro operation at stations has always been a particular challenge and, long before ATO appeared in the late 1960s, systems were developed to minimize the impact when a train delayed too long at a station.
To provide a frequent train service on a metro, dwell times at stations must be kept to a minimum. In spite of the best endeavors of staff, trains sometimes overstay their time at stations, so signaling was been developed to reduce the impact on following trains. To see how this works, we begin with an example (left) of a conventionally signaled station with a starting Signal A1 (green) and a home Signal A2 (red) protecting a train (Train 1) standing in the station. We can assume mechanical ATP (train stops) is provided so the overlap of Signal A2 is a full speed braking distance in advance of the platform. As Train 2 approaches, it slows when the driver sees the home Signal A2 at danger. Even if Train 1 then starts and begins to leave the station, Signal A2 will remain at danger until Train 1 has cleared the overlap of Signal A1. Train 2 will have to stop at A2 but will then restart almost immediately when Signal A2 clears. This causes a delay to Train 2 and it requires more energy to restart the train. A way was found to allow the second train to keep moving. It is called multi-home signaling.
Where multi-home signaling is installed at a station (left), it involves the provision of more but shorter blocks, each with its own signal. The original home signal in our example has become Signal A2A and, while Train 1 is in the platform, it will remain at danger. However, Block A2 is broken up into three smaller sub-blocks, A2A, A2B and A2C, each with its own signal. They will also be at danger while Train 1 is in the platform. Train 2 is approaching and beginning to brake so as to stop at Signal A2A. When Train 1 begins to leave the station, it will clear sub-block A2A first and signal A2A will then show green. Train 2 will have reduced speed somewhat but can now begin its run in towards the platform.
At this next stage in the sequence, we can see (left) that Train 1 has now cleared two sub-blocks, A2A and A2B, so two of the multi-home signals are now clear. Note that the starting signal is now red as the train has entered the next block A1. Train 2 is running towards the station at a reduced speed but it has not had to stop.
When Train 1 clears the overlap of signal A1, the whole of block A2 is clear and signal A2C clears to allow Train 2 an unobstructed run into the platform.
Fixed block metro systems use multi-home signalling with ATO and ATP. A series of sub-blocks are provided in the platform area. These impose reduced speed braking curves on the incoming train and allow it to run towards the platform as the preceding train departs, whilst keeping a safe braking distance between them. Each curve represents a sub-block. Enforcement is carried out by the ATP system monitoring the train speed. The station stop beacons still give the train the data for the braking curve for the station stop but the train will recalculate the curve to compensate for the lower speed imposed by the ATP system.
In addition to providing an automatic station stop, ATO will allow "docking" for door operation and restarting from a station. If a "driver", more often called a "train operator" nowadays, is provided, he may be given the job of opening and closing the train doors at a station and restarting the train when all doors are proved closed. Some systems are designed to prevent doors being opened until the train is "docked" in the right place. Some systems even take door operation away from the operator and give it to the ATO system so additional equipment is provided as shown left. When the train has stopped, it verifies that its brakes are applied and checks that it has stopped within the door enabling loops. These loops verify the position of the train relative to the platform and which side the doors should open. Once all this is complete, the ATO will open the doors. After a set time, predetermined or varied by the control centre as required, the ATO will close the doors and automatically restart the train if the door closed proving circuit is complete. Some systems have platform screen doors as well. ATO will also provide a signal for these to open once it has completed the on-board checking procedure. Although described here as an ATO function, door enabling at stations is often incorporated as part of the ATP equipment because it is regarded as a "vital" system and requires the same safety validation processes as ATP. Once door operation is completed, ATO will then accelerate the train to its cruising speed, allow it to coast to the next station brake command beacon and then brake into the next station, assuming no intervention by the ATP system
To prevent trains from running too fast. To prevent collisions between trains and buffer stops. To safeguard the movement of trains through points. To maintain a safe distance between following trains on the same track. Preventing the train to switch "mode" when not appropriate.
To adapt metro signaling to modern, electronic ATP, the overlaps are incorporated into the block system. This is done by counting the block behind an occupied block as the overlap. Thus, in a full, fixed block ATP system, there will be two red signals and an unoccupied, or overlap block between trains to provide the full safe braking distance, as shown here (click for full size view). As an aside, remember that, although I have shown signals here, many ATP equipped systems do not have visible line side signals because the signal indications are transmitted directly to the driver's cab console (cab signaling). On a line equipped with ATP as shown above, each block carries an electronic speed code on top of its track circuit. If the train tries to enter a zero speed block or an occupied block, or if it enters a section at a speed higher than that authorized by the code, the on-board electronics will cause an emergency
brake application. It was a simple system with only three speed codes normal, caution and stop. Many systems built since are based on it but improvements have been added.
A train on a line with a modern version of ATP needs two pieces of information about the state of the line ahead - what speed can it do in this block and what speed must it be 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 authorised speed code for this block and the target speed code for the next block. The diagram below shows how this works.
In this example (left), a train in Block A5 approaching Signal A4 will receive a 40 over 40 code (40/40) 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 40/25 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 25/0 because the next block (A2) is the overlap block and is 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 (0/0)
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.
Trains operating over a line equipped with ATP can be manually or automatically driven. To allow manual driving, the ATP codes are displayed to the driver on a panel in his cab. In our example below, he would begin braking somewhere around the brake initiation point because he would see the 40/25 code on his display and would know, from his knowledge of the line, where he will have to stop. If signals are not provided, the signal positions will normally be indicated by trackside block marker boards to show drivers the entrances to blocks.
If the train is installed with automatic driving (ATO - Automatic Train Operation), brake initiation for the reduced target speed can be by either a track mounted electronic "patch" or "beacon" placed at the brake initiation point or, more simply, by the change in the coded track circuit. Both systems are used by different manufacturers but, in both, the train passes through a series of "speed steps" to the signaled stop. When the first train clears Block A1, the codes in Blocks A2, A3 and A4 will change to the next speed up and any train passing through them will receive
immediately a new permitted speed and a new target speed for the next block. This allows an instant response to changing conditions and helps to keep trains moving.
The next stage of ATP development was an attempt to eliminate the space lost by the empty overlap block behind each train. If this could be eliminated, line capacity could be increased by up to 20%, depending on block lengths and line speed. In this diagram, the train in Block A1 causes a series of speed reduction steps behind it so that, if a following train enters Block A6, it will get a reduced target speed. As it continues towards the zero speed block A2, it gets a further target speed reduction at each new block until it stops at the end of Block A3. It will stop before entering Block A2, the overlap block. The braking curve is shown here in brown as the "standard" braking curve.
To remove the overlap section, it is simply a question of moving the braking curve forward by one block. The train will now be able to proceed a block closer (A5 instead of A6) to the occupied block, before it gets a target speed reduction. However, to get this close to the occupied block requires accurate and constant checking of the braking by the train, so an on-board computer calculates the braking curve required, based on the distance to go to the
stopping point and using a line map contained in the computer's memory. The new curve is shown in blue in the diagram. A safety margin of 25 meters or so is allowed for error so that the train will always stop before it reaches the critical boundary between Blocks A2 and A1.
Both the older, speed step method of electronic ATP and "distance-to-go" require the train speed to be monitored. In Fig 8 above, we can see the standard braking curve of the speed step system always remains inside the profile of the speed steps. The train's ATP equipment only monitors the train's speed against the permitted speed limit within that block. If the train goes above that speed, an emergency brake application will be invoked. The standard braking curve made by the train is not monitored. For the distance-to-go system, the development of modern electronics has allowed the brake curve to be monitored continuously so that the speed steps become unnecessary. When it enters the first block with a speed restriction in the code, the train is also told how far ahead the stopping point is. The onboard computer knows where the train is now, using the line "map" embedded in its memory, and it calculates the required braking curve accordingly. As the train brakes, the computer checks the progress down the curve to check the train never goes outside it. To ensure that the wheel revolutions used to count the train's progression along the line have not drifted due to wear, skidding or sliding, the on-board map of the line is updated regularly during the trip by fixed, track-mounted beacons laid between the rails.
Distance-to-go ATP has a number of advantages over the speed step system. As we have seen, it can increase line capacity but also it can reduce the number of track circuits required, since you don't need frequent changes of steps to keep adjusting the braking distance.
