Basics of Signalling

Signalling is one of the most important parts of the many components which make up a railway system. Train movement safety depends on it and the control and efficient management of trains depends on them. Over the years many signalling and train control systems have been evolved. The journey started with very simple systems such as simple coloured flags and semaphore arms to that today a highly technical and complex electrical and electronic systems. Here is an attempt to explain, in simple terms, how railway signalling really works.

Why Signalling is required?

In road transportation the direction and speed of a vehicle are controlled by the driver and the different vehicles share the same way at the same time in both the directions. However in Rail transport the the driver controls only the start / stop and speed of the train and the direction is controlled by the track components themselves. There is no steering wheel. One more factor to be considered is that the trains are very large vehicles and hence need large distances to increase and decrease speed i.e to start and stop also. Hence they need to be separated by considerable distances while traveling behind one another.

Thus the signaling has the following basic functions :-

1. Arranging safe reception and dispatch of trains onto required lines at stations.

2. Ensuring that trains are not received on occupied lines

3. Ensuring that two trains donot enter the same part of the track between two stations (Also called block signaling)

4. Optimizing the utilization of track and other assettes by allowing the dealing of maximum no of trains at highest speeds permitted by track and train vehicles safely.

5. Ensuring that no part of the train is left over in Block section between two stations (Optional)

6. Achieve all the above in a manner called ” Fail safe” which makes signalling a unique field of Engineering as every component and particularly the combination shall not fail to an unsafe end result at any cost.

History of Pain:

Back in the 1830s and 40s in the very early days of railways there was no fixed signalling – no system for informing the driver of the state of the line ahead. Trains were driven “on sight”. Drivers had to keep their eyes open for any sign of a train in front so they could stop before hitting it. Very soon though, practical experience proved that there had to be some way of preventing trains running into each other.  Several unpleasant accidents had shown that there was much difficulty in stopping a train within the driver’s sighting distance.  The problems were inexperience, bad brakes and the rather tenuous contact which exists on the railway between steel wheel and steel rail for traction and braking.  The adhesion levels are much lower and vehicle weights much higher on railways than on roads and therefore trains need a much greater distance in which to stop than, say, a motor car travelling at the same speed.  Even under the best conditions, it was (and is even more so nowadays with high speeds) extremely difficult to stop the train within the sighting distance of the driver.

The Time Interval System

In the early days of railways, it was thought that the easiest way to increase the train driver’s stopping distance was to impose time intervals between trains.  Most railways chose something like 10 minutes as a time interval.  They only allowed a train to run at full speed 10 minutes after the previous one had left.  They ran their trains at a 10 minute “headway” as it is called.

Flag System: Red, yellow and green flags were used by “policemen” to show drivers how to proceed.  A red flag was shown for the first five minutes after a train had departed. If a train arrived after 5 minutes, a yellow caution signal was shown to the driver. The full-speed green signal was only shown after the full 10 minutes had elapsed.

The “time interval system”, in trying to use a headway to protect trains, created some serious problems.  The most serious was that it was still inherently dangerous.  Trains in those days were considerably less reliable than they are today and often broke down between stations.  It also could not be guaranteed that the speed of the first train would be sufficient to prevent the second catching it up. The result was a series of spectacular rear-end collisions caused, in each case, because the driver believed he had a 10 minute gap ahead of him and had little or no warning if there was an erosion of that 10 minutes.  Even if the time was reduced so much that he could see the train in front, he often did not have enough braking capacity to avoid a collision.

Line Capacity:

Another serious problem, from the railways’ point of view, was line capacity.  Even if they could rely upon all trains not to make unscheduled stops and to travel at the same speed, the 10 minute time interval restricted the number of trains which could run per hour (in this case 6) over a given line.  As they needed to run more trains, they gradually began to reduce the time between trains.  As they reduced the time, or “headway”, the number of trains per hour increased.  At the same time too, the number of accidents increased.  Eventually, they had to do something.  The answer was fixed signalling.

Fixed Signalling:
Even with the time interval system, the basic rule of signalling was to divide the track into sections  and ensure that only one train was allowed in one section at one time. This is still good today. Each section (or block section as it is often called) is protected by a fixed signal placed at its entrance for display to the driver of an approaching train. If the section is clear, e.g. there is no train in it, the signal will show a “Proceed” indication. For many years it was usually a raised semaphore arm. There are a few of these left around the country but nowadays it is usually a color light or “aspect”, as the railways call it. If, however, the section is occupied by a train, the signal will show a “Stop” indication, usually a red aspect. The next train will be made to wait until the train in front has cleared the section. This is the basis upon which all signalling systems are designed and operated.

Early Signalling in India:

The List system of interlocking (named for G H List) for signalling was introduced in 1892 at six single-line crossing stations of the North Western Railway. These employed a detector and locking system for protecting facing points. The system was enhanced by A Morse and came to be known as List & Morse interlocking.

Lower Quadrant :

In IR’s lower quadrant system (Two-aspect Lower Quadrant) the semaphore arm can only be in two positions. The horizontal ON position shows the most restrictive indication (requiring the train to stop or slow down or proceed with caution depending on the kind of signal), and a lowered position (OFF) where the semaphore arm is at about 60 degrees or more from the horizontal shows the clear or proceed indication allowing a train to go past the signal.

Fig-1 Semaphore Signals that stoop (lower quadrant)

Upper Quadrant

Properly, Multiple Aspect Upper Quadrant, in this system there are three signal positions. The 12 o’clock position is clear or proceed, which gives a train permission to go past the signal without stopping. An intermediate position (at 45 degrees to the vertical) is the attention or caution indication; the meaning depends on the kind of signal. The horizontal position, where the semaphore arm is horizontal, the onposition, is the most restrictive indication of the signal; it may require the train to stop, or to proceed with caution, etc., depending on the kind of signal.

Fig-2 Semaphore Signals that raise (upper quadrant)

Nowadays IR has converted what are known as Color Light Signals (CLS) with Multiple aspects where the color of Light indicates meaning to Drivers ( Now called Loco Pilots).

 

Fig-3: Multiple aspect Color Light Signals
Two-aspect colour-light signalling – In this, each signal has two lamps (one above the other). The higher of the two is a green lamp, and the lower one is a red lamp. The green lamp when lit indicatesclear (the proceed indication), and when the red lamp is lit, the signal is said to be in the on position, displaying its most restrictive indication.
Three-aspect colour-light signalling – In this, each signal has three lamps arranged vertically. The top one is green, the middle one yellow, and the bottom one is red. The red and green lamps indicate indications as in the 2-aspect system, and the yellow lamp shows the caution indication.
Four-aspect colour-light signalling – This is also known just as Multiple-aspect colour-light signalling (MACL or MACLS) and adds another yellow lamp to the 3-aspect system. The additional yellow lamp can be placed above the green lamp in a 4-lamp signal. In this case, the lower yellow lamp alone is lit to show the caution indication, and both yellow lamps are lit to show the attention indication. Alternatively, a different kind of 3-lamp signal may be used (e.g., for distant signals), where the top and bottom lamps are yellow and the middle one is green. Again, both yellow lamps light up to indicate the attentionindication.

Meanings of Aspects :

FIg-4 : Aspects (Appearance to Driver) and Meanings
Stop This requires a train to stop dead and not pass the signal except under special instructions or emergency procedures. (Stop signals may be passed after halting and waiting in automatic block territory – usually 1 min. during the day & 2 min. during the night.) This indication is also known as Danger.
Caution This allows a train to proceed past the signal with caution (at reduced speed), being prepared to stop at the next signal. It can mean that the next signal is at Danger, or that the track ahead has speed restrictions.
Attention This allows a train to proceed past the signal, being prepared to slow down to an appropriate speed for the next signal. It means that the next signal may be at Caution, or may guard a divergence which requires reduced speed (in which case a stop signal at the divergence will indicate the route for which points are set).
Proceed This allows the train to proceed past the signal without slowing down or stopping.

Why Signalling is required?

In road transportation the direction and speed of a vehicle are controlled by the driver and the different vehicles share the same way at the same time in both the directions. However in Rail transport the the driver controls only the start / stop and speed of the train and the direction is controlled by the track components themselves. There is no steering wheel. One more factor to be considered is that the trains are very large vehicles and hence need large distances to increase and decrease speed i.e to start and stop also. Hence they need to be separated by considerable distances while traveling behind one another.

Thus the signaling has the following basic functions :-

1. Arranging safe reception and dispatch of trains onto required lines at stations.

2. Ensuring that trains are not received on occupied lines

3. Ensuring that two trains donot enter the same part of the track between two stations (Also called block signaling)

4. Optimizing the utilization of track and other assettes by allowing the dealing of maximum no of trains at highest speeds permitted by track and train vehicles safely.

5. Ensuring that no part of the train is left over in Block section between two stations (Optional)

6. Achieve all the above in a manner called ” Fail safe” which makes signalling a unique field of Engineering as every component and particularly the combination shall not fail to an unsafe end result at any cost.

To achieve the above functions the follwing devices are used :

1. Track circuits : are simple electric gadgets that are filtted to tracks and detect the presence of trains over that portion of the track. They prevent allowing of signals on the same portion by fixing the signals at Danger (RED) position till such time the trains leaves that portion. Thus this gadget allows dealing of trains without colloisions.

Fig-5 Track Circuit – General

Fig 5a – The Track Circuit – Without Train (Signal can turn Green)

Fig 5b – The Track Circuit – Occupied by train (Signal goes to Red)

Axle Counter : Has electromagnetic devices fitted on track which count axles (wheels) of trains rolling in and rolling out. When both counts  match, the section is considered free of a train :

Fig 6 – The Functioning of Axle Counters

  • Other Components :
  • Point machines ( to change points; read below about points)
  • Relay or Electronic Interlocking for correlating all field gears before clearing signals)
  • Panels with yard diagram for taking orders from Station master.
  • Block Instruments for ensuring that two trains donot enter the space between two stations in an unsafe manner.
  • Lifting barriers to ensure road vehicles are not allowed during train movements
  • Signals of different types to inform driver to move or stop.
  • Dataloggers to monitor correct sequences and pre warning or analyzing unsafe outcomes
  • Automatic Signalling which works without humanintervention in busy sections esp in suburban transport
  • Advanced Train wraning and train protection systems
  • Powersupply systems to support reliable and safe working of Signalling

Signalling at Stations :-

We are aware that though there are only one or two lines between stations (called block section); at stations there are many lines onto which the trains are allowed to be received and dispatched. We have also learned that a train driver cannot steer his train in the required direction. Thus railway is called guided transportation. The track itself modifies its components dynamically to lead the train to required line (platform). An important part of the track that achieves routing of trains is a point.

Point also called Turnout

Fig 7a: Point : also called a switch is a device on the track that Guides a train into any given Platform or Track-line at a Rly. station, in the above picture the point is set for the line on the RHS ….. (Click Picture to see animation)

Photo of a Point :

All such points and connected line at a station is known as a yard. A typical simplified signalling diagram is indicated below:

Fig 6 : Yard Signalling sketch of a small station on Double Line (Double Line Means seperate tracks between stations for UP and DOWN direction trains).

The Field gears in Electrical Signalling Installations ( which has been the order of the day) called Panel Interlocking or Electronic Interlocking/ Route Relay Interlocking is controlled  by Station Masters using an operation cum indication Panel.

FURTHER ROLE OF SIG/TELECOM/IT:  

Passenger amenities
Manufacturing of specified signalling items
Providing telecommunication infrastructure for :
Safe and efficient operation of trains
Administrative voice and data circuits
Passenger Reservation System (PRS)
Unreserved Ticketing System (UTS)
Freight Operation Information System (FOIS)
Crew Management System (CMS)
Coaching Operating Information System (COIS)
Management Information System (MIS)
Management of Railways Corporate Wide Information System (RAILNET)

Advanced Signalling Technologies : (Update 19-12-2011

Beyond Normal Signalling the Following Areas of Signalling are adding to  Increase in Traffic and Train Safety :

1. Automatic Signalling :

Though it is a Traditional Technique where the section between two  Stations is Split into Multiple Sections of About 1 KM  with Signals Controlled by Trains themselves with Track Circuits without Human intervention. There will be Multiple Trains between any two stations seperated by about 1 Km and protected by Aspects of Signals. The Latest in this Field has been the use of Multi Section Digital Axle Counters made by Companies like  Alstom .

2. AFTC : Audio Frequency Track Circuit :

To reduce Dependance on Track insulation Joints and to increase the range of Normal Track Circuits (which were limited to 350-450 Mts) AFTCs have been developed. The track rails/Sleepers are put into a tuned Audio Frequency Circuit capable of detecting train presence without need for Insulated rails.

3. MSDAC : As already mentioned earlier MSDAC (Multi section Digital Axle Counters offer a new Alternative to Track Circuiting. The use of MSDAC is a means to avoid dependance on Electrical Parameters of Track and Rail Insulation Joints and has been found to a very useful Technology in Yards.

4. TPWS : Train Protection Warning Systems are combination of on Track and Loco Mounted Devices linked to Field Signalling Gears to not only Pre-Warn the drivers about Signals, Caution Driving notices and other Limitations in advance but also to apply breaks if the Driver neglects the same. This is the only solution today for trains going as per Schedules in Foggy Environments apart from being basic Safety Tools in High Density Sections.

5. Cab Signalling : An Extension of TPWS ( The EU ETCS Level-3) is a device to avoid track side Signalling and the Signals are Virtual and are Directly reported to Driver and track side  Physical Signals are Eliminated.

6. Moving BlocK: is a further Extension of the above Signalling where in not only the Track side Signals are made redundant but also tey are made virtual and movable (not geographically fixed and moved as per traffic to maximise Traffic density and safety).

7. Electronic Interlocking : The traditional Station Signalling in the Last Few Decdes has been Relay INterlocking. In the Last decade the Computerized Interlocking or Electronic Interlocking has been taking precedence. The Basic issue with use of Electronics for Rail Siganlling Systems is the Concept of ” Fail Safe ” Techniques. The fail safety was easily built into Mechanical and Relay Based Systems but Electronics by nature is not Fail safe and the same has to be achieved by Redundancy Techniques in hardware and software. All Rail Siganalling systems have to meet SIL-4 (System Integrity Level-4) of CENELEC a European Electronic Standardization Agency.

RDSO Railway Designs and Standrads Organization ( Similar to TEC of DoT) is the Organization that standardizes IR Signalling safety issues and Eqpt Specifications.

8. OFC Network : Mostly maintained by Rly PSU, M/S Railtel Corporation has adavance backbone features such as DWDM and is an Emerging Player in building India’s Back bone network apart from serving Rly Telecom needs.

Updates will Follow .

A Detailed Write up on the subject :

Signaling &Telecom Systems on IR

15 comments on “Basics of Signalling

  1. It is surprising that despite managing multiple MIS of IR, the S&T had not developed an MIS for itself. Whats the updates???

    • MIS is there for S&T but somehow the the ” Failures” mind set at the Decision making level is shifting focus away from the IT enhancements and leadership in IT initiatives by IRSSE. Lets hope things will change for better. The PRS denial was a Historical mistake which doesnt seem to have changed over a time.
      -IRSSE

    • MIS was implemented in SR and then in some other Rlys like SCR (same version as SR) which is not Web Based and very Rigid shall we say Stupid.

      It was stressed upon in 2008 and lots of data force fed then only to be left un-updated latter.

      IR doesnt have a proper MIS. It cannot work until and unless it is linked with Day to day Operations and monitoring.

      • Hello Sir,
        I heard that Signalling Engineer is a field job ..and not much secure for girls..is it true…What is the most suitable job in cadre A for girls in railways…

      • In Grp-A its OK but You have to come out of the woman mindset and be bold .. Nothing will happen .. But pl dont expect too many concessions on the basis of Gender …. This the personal opinion of Web Master as the only brother of five sisters and father of two lovely daughters… Even my M-in-Law lives with us and treats me as a a son though her “son” is in the same city and is not able to even visit her… Pl take it in the right spirit .. IRSSE

      • Definitely not best suited for Girls … Unless You can walk on the track at any part of the day or night , you shouldnot choose it … But choice is yours … IRSSE

  2. I had proposed for developingthe MIS for S&T when I was the Dy.CM-IT/SECR. Now, with my transfer, I am not that much sure of its development, though I will try.

  3. I am tranferred as SrDSTE/R Division. I will try to find time to set the base for developing a ral good MIS, probably named SNTIPDE-SIGNALING, NETWORK & TELECOM’s INFORMATION, PROCESSING & DOCUMENTATION ENGINEERING”. I need all the guidance, pointers and suggestions to start on this. Hope that our Directorate will also support & help towards development in this field.

  4. I hope you will get support from everybody and will create an excellent SNTIPDE. Your skills in this field and your success on this in SECR are legendry. SECR had managed and is managing verywell with the MIS and Central srvers commisioned by you, and its upgrade, also duly designed by you are on the way now. I sincerely wish that your great succcess there will be copied in-toto here also

  5. i am a contractor in railway. and in a search of a retaired railway signalling person. i stay in ahmedabad.

  6. Before time operating is totally depend on S&T department.but now at this time’s&T is totally depend on operating …..? …why…….
    S&T is vital role play in Indian railway…..
    For safety or pantualti …..naturally answer is safety.??????????????????

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