The system adopted for controlling the movement of trains should be such that it allows the trains to run in either direction as well as facilitates faster trains to overtake slower trains, thus ensuring the complete safety of trains. The following systems are chiefly used for controlling the movement of trains on Indian Railways.
Time interval system
In this system, there is a time interval between two successive trains. A train is dispatched only after sufficient time has elapsed since the departure of the previous train. This system works fine just as long as everything goes well with the previous train, but if there is a mishap and the previous train is held up, the system fails, jeopardizing the safety of the trains.
Space interval system
In this system, there is a space interval between two consecutive trains. Only one train is permitted to occupy a particular length of the track. A succeeding train is permitted to occupy the same track length from either side only after the first train has cleared it. This system guarantees safety as only one train is in motion at one time.
Methods of Controlling Train Movement
Based on these systems, the following methods are adopted for controlling the movement of trains on Indian Railways.
This system permits only one train to remain in a section at one time. The movement of trains is controlled with the help of a wooden staff or a token with suitable identification marks, which are in the possession of the driver of the train. As the same object cannot be at two places at the same time, the safety of trains is fully ensured. This system is possible only on short branch lines that have limited traffic. Normally there is only one train, which works to and fro on the same section. The system fails if it becomes necessary to dispatch more than one train in the same direction. This system does not require a 'line clear' directive.
In this system, trains follow each other after a time interval that is generally less than 15 min. Trains scheduled after the first train can run at a maximum speed of 25 km/h. As an adequate time interval is kept between two successive trains, safety is ensured to a limited extent. The system is used under the following circumstances.
(a) In the case of emergencies such as the failure of block instruments and the telephone system
(b) In short double-line stretches
Pilot guard system
In such a system, one person, known as the pilot guard, accompanies a train by riding on the foot plate of the engine (or gives a ticket personally to the guard of the train, which is authority to proceed) and returns to the same station with another train. The pilot guard is normally identified by his or her prescribed uniform, which is red in colour, or the badge that he or she wears and is an authority for the train to proceed. Even in this system trains can follow each other after a fixed time interval of not less than 15 min. The system is applicable in short single-line sections or in the case of failure of communication between two stations.
Train staff and ticket system
This system is similar to the pilot guard system. The authority to proceed in this case is either a wooden staff or a ticket. There is only one wooden staff for a section and the same is kept at one of the two stations on that section. Each station has a ticket box which contains printed tickets and is kept locked. The wooden staff is interlocked with the box in a way that it cannot be taken out so long as the box is locked. A train can only be dispatched from the station that has the staff. In case only one train is to leave the station, then the staff is handed over to the driver of the train. If more than one train is to be dispatched from the same station, the preceding trains are dispatched on the authority of the ticket while the last one is dispatched along with the staff. The time gap between two successive trains is not less than 15 min and the speed of the trains is restricted to 25 km/h. A similar system is followed for dispatching trains from the other station. In this system, the safety of the trains is ensured on account of the fact that only one ticket can be issued at one time and the driver insists on seeing the staff before accepting the ticket as his authority to proceed.
Absolute block system
This system involves dividing the entire length of the track into sections called block sections. A block section lies between two stations that are provided with block instruments (explained later). The block instruments of adjoining stations are connected through railway lines and a token can be taken from the block instrument of a particular station with the consent of both the station masters.
In the absolute block system, the departure of a train from one station to another is not permitted until and unless the previous train has completely arrived at the next station, i.e., trains are not permitted to enter the section between two stations at the same time. The procedure by which this system is maintained is known as the lock and block procedure. The instruments used for this purpose are known as block instruments.
Block instruments Each station has two block instruments; one for the station ahead and the other for the previous station. The block instruments of two adjacent stations are electrically interconnected. These block instruments are operated with the consent of the station masters of the stations on either end of the block section, who are also responsible for giving the line clear indication. Normally a round metal ball called a 'token' is taken as the authority to proceed in a block section. This token is contained inside the block instrument.
There following different types of block instruments are used on Indian Railways depending upon various requirements.
Single-line token instruments These are meant for stations with single lines. No train is authorized to enter the block section without a token. The token can be taken out of the block instrument of the departure station only when the station master turns the handle of the block instrument towards the end labelled 'Train going to side'. This can be done only with the consent of the station master of the station on the other side of the block section, who turns the handle of his or her block instrument towards the end labelled 'Train coming from side'. It is not possible to turn the signal permitting the entry of the train into the block section off until the handle of the block instrument has been turned towards the 'Train going to side' label. In this situation, the handles of both these instruments get locked in the last operated position and it is not possible to normalize both the block instruments until the train arrives at the next station and the token has been inserted into the block instrument of that station. This phenomenon of keeping the block instruments locked and releasing them only during the passage of a train is the previously mentioned lock and block procedure.
Single-line tokenless block instruments There have been occasions when a train has had to be brought to a halt because of the driver misplacing the token, causing the trains to get detained for long periods. In order to avoid such occurrences, tokenless block instruments have been developed. The same principle as that of the block system is followed here but without the use of a token. The last stop signal permitting the entry of the train into the block section, which is normally the advanced starter signal, is interlocked with the block instrument in such a way that it is not possible to turn this signal off unless the block instrument has obtained the line clear command.
Double-line block instruments In a double-line section, traffic is unidirectional. The block instrument comprises of a commutator handle and two indicator needles placed in vertical alignment. In order for the block instrument to work on a double line, the station master turns his block instrument commutator to the 'Line clear side'. This causes the electrical circuit to make contact in such a way that the advanced starter of the dispatching station can be turned off.
Working details Take an example of a block section AB situated between two stations A and B on a single-line section (Fig. 31.20). A train is waiting at A to enter the section AB. The procedure is as follows.
1. The station master of station A establishes telephonic contact with the station master of station B with the help of the block instrument and requests the station master of station B to grant a line clear, i.e., permission so that he can dispatch train A.
Fig. 31.20 Block section AB between stations A and B
2. Once the station master at station B has ensured that the line is clear according to the prescribed norms, he agrees to receive the train and grants a line clear. For this, he gives a private number and operates the block instrument of his station in a prescribed manner. The station master at station A notes this private number and simultaneously operates his block instrument so that a 'ball token' is extracted from the block instrument.
3. The station master at station A then allows the point to be set, lowers the signal and hands over the 'ball token' to the driver of the train waiting at station A.
4. The station master at station B also gets the points set and lowers the signal for the line on which the train is to be received.
5. The train then starts from station A and enters the block section AB.
6. The train reaches station B. The driver of the train hands over the ball token to the station master of station B. After ensuring that the entire length of the train has been received, the station master at B pockets the ball token in the block instrument. He then informs the station master at station A of the arrival of the train on a private number as proof of the same. The points at station B are then set as they were before and the reception signals restored to their normal positions.
7. The same procedure is repeated when the train has to enter a block section BC.
The system is absolutely safe and works on the principle of providing space intervals. Most stations on Indian Railways work on this principle. The following are the essential features of the absolute block system.
(a) No train is allowed to leave a station unless 'permission to approach' has been received in advance from the block station.
(b) On double lines, permission to approach is not given until the line is clear, not only up to the first stop signal of the next station, but also for an adequate distance beyond it.
(c) On a single line, 'permission to approach' is not given until the following conditions are satisfied.
(i) The line is clear of trains running in the same direction, not only up to the first stop signal of the next station, but also for an adequate distance beyond it.
(ii) The line is clear of trains running in the opposite direction.
(d) When two trains are running in the same direction on the same track, permission to approach should not be given to the second train till the entire length of the first train is within the limits of the home signal, the 'on' status of all the signals behind the first train has been restored, and the line is clear, not only up to the first stop signal of the station, but also up to an adequate distance beyond it.
Automatic block system
In the space interval system, clearing a long block section is a protracted event and the subsequent train has to wait till the preceding train clears the entire block section. This impairs the capacity of the section with regard to the number of trains it can clear at a time. In order to accommodate more trains in the same section, the block section is divided into smaller automatic block sections. This is particularly done for sections that are long and have turned into bottlenecks. The essentials of an automatic block system on a double line are as follows.
(a) The line should be provided with continuous track circuiting.
(b) The line between two adjacent block stations may, when required, be divided into a series of automatic block signalling sections, entry into each of which will be governed by a stop signal.
(c) The track circuits should control the stop signal governing the entry into an automatic block signalling section in the following manner.
(i) The signal should not assume the 'off' position unless the line is clear in advance, not only up to the next stop signal, but also for an adequate distance beyond it.
(ii) The signal should automatically turn on as soon as the train passes it.