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.
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)
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 :
Danger Caution Attention Proceed
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)
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 …..
Fig 7b : (Click to see animation) Animated demonstration of How a Point can lead train wheels in different Directions
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).