Metro Rail Basic Signalling Concepts
Control over movement of Trains & Development of signals.
• This mode has its own inherent problems.
• Trains cannot be steered away as in the case of other transports.
• They are required to follow one another in the same direction on track, as otherwise for every vehicle separate parallel paths are to be provided.
• Control over movement of Trains & Development of signals.
• Railway locomotion, therefore, though more efficient, brings in problems of “Control over movement of Trains”.
Railways Signalling adopts two methods for controlling over movement of train:
–Time interval method
–Space interval method
Time Interval Method
• In this method, trains are dispatched in the same direction at a fixed interval of time, irrespective of the status of earlier dispatched train.
• Where the speeds and weights are low, it is not difficult for a following train to stop short of the train ahead, which has stopped.
• With higher speeds and heavier loads, the distance required to stop a train is longer, and at this longer distance, the driver cannot definitely decide whether a train in front has actually stopped or not.
• This makes it difficult for the driver of the following train to run his train at desired speed.
• If all trains run at the same speed and are required to stop at the same place for the same duration, a certain amount of control can be exercised by having a definite time lag between the trains from one stopping place to another.
But in the case of Railway, this is not practicable as –
• (a) Different types of trains like, Express/Mail, passenger, high-speed freight and low speed freight shunting trains are run.
• (b) The speeds of all the trains are not same.
• (c) The terrain of the country is not same everywhere.
• (d) The brake power, hauling capacity, load of train is not same for all trains; and
• (e) The stopping places of all trains are not the same.
• Hence, it is not possible to control the movement of trains under the “Time interval” method.
• A better method of control the “Space Interval Method” is therefore more commonly adopted.
Space Interval Method
• In this method of “Control over movement”, the length of track is divided into sections called “Blocks”. The entry of a train into the ‘block’ is controlled in such a way that only when it is free, a train can be allowed to enter it. This means that between two consecutive trains, there is a definite space interval.
`A’ and `B’ are two block stations. Under the Absolute Block System controlling the movements of train between `A’ and `B’ is such that, Station `A’ can allow a train to leave his station towards Station `B’ only when the line clear is obtained from Station `B’. Station `B’ is supposed to give the “line clear” for a train to approach towards `B’ only, when the whole block section is clear of trains. That is, when the previous train has cleared the block section as well as the adequate distance beyond the block section.
• This system of controlling train movement is more commonly known as “BLOCK SYSTEM or BLOCK WORKING ”.
• So, with the two controlling points and intercommunication, it is possible to control the entry of a train into a block only when it is vacant.
Signaling in Metro Operations
• Such constraints of railway working is observed more prominently in metro system of working as:
• The frequency of trains (Headway) is very high.
• Availability of lines for operations on main line are very limited (only one line for up direction and only one for down direction).
• The crossing stations are limited and as the trains are worked in a closed loop, a delay to one train is potentially a delay to entire system.
• Such challenges can only be met by adopting the most advance technology of railway signaling for working of trains on a metro system.