Modern Signalling Installations

Advancements in electrical engineering and electronics have greatly contributed to the modernization of signalling installations, leading to better safety, increased speeds, and quicker movement of trains. Some of these modern signalling installations are described below.

Panel interlocking

In panel interlocking, all points and signals are operated electrically from a central location and the switches for operating these points and signals are mounted on a

panel, which also bears the diagram of the yard layout. Electrical interlocking of these points and signals is achieved by means of relays. The main advantage of panel interlocking is that the various functions of all the points and signals, even though the same cover great distances, can be centrally controlled, thereby eliminating the need for multi-cabin operations of the same. With the elimination of intercabin control and slotting, the time that is normally lost in coordination is saved and the line capacity increases so that a greater number of trains can be run by a smaller operating staff.

Route relay interlocking

Route relay interlocking is an improvement over panel interlocking. Unlike panel interlocking, where each point in the route has to be individually set with a respective switch and where the clearance of the signal is obtained by operating the signal switch, route relay interlocking involves the use of only a pair of switches to perform all these operations automatically. Using this pair of switches, the desired route for the train is set automatically by putting all the points along the route in their desired positions. The required signal is then cleared automatically too.

During this operation, it is also ascertained that there is no conflicting movement of the trains in progress or in the offing and also that the route is clear for the movement of a train, including at the overlap. One of the essential requirements for this is that the entire yard has got to be track circuited. The condition of the track circuits and the various indications of all the signals on the route are mirrored on the panel that carries the diagram of the yard. By looking at these indications, the panel operator can easily discern which portion of the track is clear or occupied and which signal has been cleared for the movement of the train. Once the route is set to allow the passage of a train, the relevant portion of the diagram on the panel gets illuminated with white lights. The lights turn red when the track is occupied by the train. When the train has cleared the track, the lights automatically go off.

Route relay interlocking is very useful in busy yards such as the Mumbai suburban section, where traffic density is very heavy. As route relay interlocked yards are fully track circuited, they ensure complete safety with regard to the movement of trains.

Centralized train control

The operation of all the points and signals of the various stations of a section is centralized at one place in such a system. Thus all the points and signals are controlled by a single official called the centralized train control (CTC) operator. A CTC operator virtually takes over the work of the station masters of several individual stations and operates all the points and signals at a station through remote control.

The CTC panel is normally provided at a central location and controls various stations up to a distance of about 120 km on either side. There is a separate panel provided for the operator, which depicts the entire section, including the points, crossings, signals, etc. The signals, routes, points, etc., are operated from the panel by means of separate knobs. This panel also depicts whether the various tracks are occupied or otherwise.

In a CTC system, panel interlocking is provided at all stations, which ensures complete safety. The CTC operator sends commands to the station equipment in the form of coded electric pulses by pressing the relevant buttons. The station equipment receives these commands, and sets the points to the desired position, and clears the appropriate signals. After the task is completed, indication signals are automatically sent back to the CTC panel in the form of coded electric pulses and the positions of the points and signals are indicated on the panel. In the route interlocking system, instructions regarding the running of trains, arrangement of crossings, etc. are issued by the control office through phone calls placed to various station masters and the actual control of the movement of the trains between the stations is exercised by the station masters. In the CTC system, all the functions of the controller and the station masters are carried out by the CTC operator, who is always aware of the position of all the trains in the section through the illuminated panel and who can remotely operate the various signals and points at all the stations. He or she can, therefore, make judicious plans regarding how the trains will move forward and how they will cross each other. Moreover, the automatic block system is always adopted in conjunction with CTC with the result that the number of trains in a block section can also be increased. The major advantages of the CTC system are enumerated below.

(a) There is considerable saving in the amount of time taken by trains to complete a run and as such the line capacity of the section is increased. In fact, with the introduction of CTC the necessity of doubling the track can be overlooked.

(b) No trained station masters, points men, etc. are required at the various stations. The CTC operator does all the work from the central panel. Thus there is considerable reduction in the number of skilled staff members required.

(c) The system has the potential to detect any defects in the track.

Automatic warning system

Any amount of safety features incorporated in signalling and interlocking equipment will be of no use if the driver ignores a danger signal. The automatic warning system (AWS) is a device that triggers the automatic application of brakes if the signal is indicating danger and the driver has not taken any action. The system consists of a track device located at a desirable braking distance at the rear end of the first stop signal. The track device is activated when the signal indicates danger and is ineffective when the signal is 'clear'. When the locomotive passes over the track device with the signal indicating that the line ahead is clear, nothing happens. However, if the signal happens to be indicating danger, a red lamp gets lighted as soon as the locomotive passes over it followed by the ringing of an alarm bell. If the driver presses the acknowledgement button and applies the brake, the alarm stops ringing and no further action is taken by the AWS. If, on the other hand, no action is taken by the driver, the bell continues to ring. The emergency brakes are then applied automatically and the train is brought to a stop.

Last vehicle check device

One of the important features of the absolute block system is that a train should not be allowed to enter the block section unless the last train has arrived at the station

from either end of the block section. This must be verified by the operator responsible for operating the block instrument by observing the last vehicle board or last vehicle light. It has been observed that in quite a few cases the operator has been found careless in certifying the last vehicle. To eliminate human error, a device known as the last vehicle check device has been developed. This consists of a passive equipment that the guard hangs on the last vehicle of the train. Each station is provided with a corresponding active equipment that emits waves of a predetermined frequency. This equipment is fitted at the entrance of each station. The coupling of the instruments takes place whenever the last vehicle passes by the active equipment, which is sensed by the last vehicle check device (LVCD). The operator receives an indication of the same and can now close the block instrument and issue a fresh line clear. The last vehicle check device eliminates human error in ensuring the safe arrival of the complete train.


Signals are used to regulate the movement of trains and interlocking ensures the safe working of signals. There are many different kinds of signals and each one of them gives vital information regarding track conditions. These signals have separate indicators for day and night. There are various methods of interlocking starting with the simple key method to the advanced automatic train control system. With advancement in signalling as well as interlocking systems, it has become possible to run trains at smaller intervals, ensuring optimum use of the track capacity.

Review Questions

1. Give the classification of signals according to their locations in station yards along with suitable sketches.

2. (a) What are the objectives of interlocking? Explain the tappet and lock system of interlocking.

(b) Briefly describe the absolute block system of controlling the movement of trains for single and double lines.

3. What essential purposes are served by signalling and interlocking? What is meant by route relay interlocking?

4. (a) Briefly describe the locations and purposes of the following signals:

(a) warner, (b) outer, (c) home, (d) starter, (e) advance starter.

(b) What are the minimum signal requirements in each direction for a two-aspect signalling system in class A, B, and C stations?

(c) What are the essentials of interlocking? Distinguish between direct and indirect interlocking.

5. (a) Explain what you understand by interlocking in a railway system.

(b) What purposes does the lock bar serve?

(c) Distinguish between the following pairs of terms.

(a) Repeating and co-acting signals

(b) Warner and distant signals

(c) Absolute block system and automatic block system

(d) Track circuit and rail circuit

6. Describe the three aspects in upper quadrant signalling. Briefly describe one method of interlocking used on Indian Railways.

7. Write short notes on (a) absolute block system (b) rudimentary interlocking, and (c) MAUQ signalling. How many standards of interlocking are in vogue on Indian Railways? What requirements must be met by each of them?

8. Briefly describe semaphore signals and coloured light signals.

9. Explain with the help of sketches the use of the compensator device in signalling with regard to the thermal expansion of wires.

10. Mention the objectives of signalling. How are signals classified? Mention the functions of each signal.


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