1.

INTRODUCTION

1.1 UNIQUENESS OF RAIL TRANSPORT:

Working of a Rail Transport is different from that of other Modes of Transports. The
Movement of a Train is Track-bound. The Railway Track is usually called the Permanent
Way and consists of two parallel flat-bottom Steel Rails (Cross-section: Asymmetrical I-
beam) anchored perpendicular to Members called Iron or Timber or Pre-stressed
Concrete Sleepers, which are themselves laid on crushed Stone Ballast. Tie-plates (or
Base Plates) are interposed between the Rail Foot and the Sleeper to spread the Load
and reduce Abrasion.

Railway Track also contains Turnouts, known as Points and Switches, which are the
means of directing a Train onto a diverging section of the Track. The Switch Rails of
Points can be moved left or right, to determine which path the Train should follow.

A Train, running on a Railway Track, can not change its course like a Car, Bus or Truck
running on the Road. There is no Steering kind of Control available in a Train. Little
edges or Flanges on the Wheels of Rail Vehicles & the Rigid Geometry of the Track,
however, keep the Vehicles from slipping off the Rails. The movement of the Train,
therefore, has to be essentially subjected to elaborate & rather rigid Methods,
Procedures & Rules to prevent Accidents. Any Infringement or Violation of these
Methods, Procedures & Rules may lead to an Accident. Possible Accidents include
Derailment (Jumping off the Track), a Head-on / Rear-end Collision with another Train
and Collision with an Automobile or other Road Vehicle at a Level Crossing.

Since the Interaction of Steel Wheels of the Train and the Steel Rails of the Track is
through a very smooth Surface (Steel to Steel) with Low Frictional Resistance, the
Trains can be made to travel at a very high Speed. Hauling heavy Loads, they shall,
however, require a larger distance to stop, which shall depend upon the Operating
Speed of the Train & the Geometry of the Track.

Keeping in view the above unique Features of the Rail Transport System of Trains
running on a Fixed Track having no Steering Control and requiring larger Braking
Distance to stop - should the running conditions demand so, the Key Aspects of running
Trains in a Rail Transportation System could be summarized as under:

(i) Keeping the Train on the Rails – Managed by (a) the Design of the Wheel
& Cross-section of the Rail, and (b) the Track Geometry and a stricter
regime for its Maintenance.

(ii) Safely Controlling the Railway Traffic to prevent the Trains from Colliding
(Head-on / Rear-end) – Managed by the Signalling Control System by
maintaining a defined Space Interval between them.

Running on Fixed Steel Rails with low Frictional Resistance, the Trains
are uniquely susceptible to Collision since they frequently operate at
Speeds that do not enable them to stop quickly or within the Driver's
 Sighting Distance and that they do not have Steering kind of Control
available in Road Vehicles. Instead the Arduous & Complex Signalling
System provides the needed Control Mechanism.

Most forms of the Train Control involve Movement Authority being passed
from those responsible for each section of a Rail Network to the Train
Crew. This could be through Signals placed alongside the Track or
through the Signals displayed on the Driver’s Console. The later form of
Signalling is called ‘Cab Signalling’.

(iii) Controlling the Speed of the Train – Managed by the Signalling System.

The Speed Control Mechanism could be built-in through the conventional

Signalling, providing necessary Directions to the Driver to follow the
Aspects displayed by Line side Signals to keep the Train under his
Control or through a partially or fully Automated Train Control System
(ATC).

The Signalling Process has been traditionally carried out in a Signal Box (i.e. the Cabin
or Panel Room), a small building that houses either the Lever Frame or a Control Panel,
required for the Cabinman / ASM / SM to operate Switches and the Signal Equipment.
These are placed at various Intervals (two ends of a Block Section) along the Route of a
Railway, controlling specified Sections of the Track. More recent Technological
Developments have, however, resulted in the Centralisation of the Signalling Operations
to Regional Control Rooms (System being usually called as Centralised Traffic Control
or the CTC). The Centralised Control has been facilitated by the increased use of
Computers, allowing vast sections of the Track to be monitored from a single Location.

1.2 ROLE & REQUIREMENT OF SIGNALLING IN RAILWAY

OPERATIONS:

Signalling, in its simplest way, can be conceived of as a form of communication between

two parties who cannot communicate with each other either orally or in the written form.
We are all familiar with the roadside Signals that a policeman uses to regulate the road
traffic. The show of hands or the exhibition of Red, Yellow or Green lights at a road
crossing Signal post are the Signals that he employs to tell the Driver of the road vehicle
to stop, exercise caution or proceed.

In Railway Operation, such communication becomes necessary between the Driver and
the Staff who control ground equipment like the Points at a station and Level Crossing
Gates etc. Signals become the link between the Driver & ground Staff to indicate correct
setting of Points & Route and closing of Level Crossing Gates against the road traffic.
Station Staff is required to set the Route and signal to the Driver of a Train to start from a
Station or enter into a Station from the Block section, as the case may be. The Gateman
in charge of a Level Crossing Gate is required to be always attentive, close the Level
Crossing Gate in event of passing of a train at a Gate and thereafter give an all-right
Signal to the Driver indicating that Gate has been closed properly and locked against the
road traffic. In case, the Gate is opened for road traffic, the Driver is to be signalled to
stop short of Level Crossing Gate. In all the above situations, the most convenient form
of communication between the Driver and ground Staff is through Signals rather than
through oral or written methods, though written communications as also oral ones are
also used at times to convey the requisite information to the Driver.

Signalling on Railways is required to perform two main functions of ensuring Safety in

Train Operations and increasing the Line Capacity of the section with minimum of the
investment.

Signalling seeks to promote safety by minimising the impact of human error on the
Safety of Train Operations and, thus, it is the most essential instrument of Safety on the
Railways. Techniques of Signalling have advanced today to an extent that it is possible
to eliminate the chances of fallibility of human element to a degree, not conceived of a
few decades ago. Application of modern Systems like Centralised operation of Points &
Signals through SSI, Automatic Signalling through continuous Track circuits or Axle
Counters, Train Protection & Automatic Warning System (TPWS), Last Vehicle Checking
by Axle Counters, Track Circuits at Stations, Interlocking of and provision of Train
Actuated Warning Devices at Level Crossing gates, OFC / Quad cabled communication
circuits, and Mobile Train Radio Communication (MTRC) etc. can greatly reduce
accident situations on Indian Railways.

On Line Capacity front, appropriate Signalling can permit large number of Trains being
run on a Section without need of increasing number of tracks, thus achieving optimum
utilisation of assets with only 10% - 15% of investment than that required for laying of an
additional track.

Line Capacity of a section, inter-alia, depends upon the largest time taken by slowest
train to cross the ruling block section as also on the average time required for operation
of Signalling gears and Block Instruments. It is this Operations' time that gets
significantly reduced by introducing automation through sophisticated Signalling systems
and this significantly contributes towards enhancement in Line Capacity. In addition,
modern Signalling Systems increase the Efficiency factor in Operations which also goes
a long way in further augmenting the Line Capacity on the Section.

1.3 CARDINAL PRINCIPLES & FEATURES OF A RAIWAY

SIGNALLING SYSTEM:

Safety of passengers and efficiency of operation being the twin objectives for which the
Signalling is installed, formulation of its principles and meticulous observance thereof is
of prime importance in designing and developing appropriate Signalling systems for a
section. Following are some of the cardinal principles of Signal Engineering, evolved
over time, which should be followed religiously in fulfilling the role assigned to Signal
Engineers on Railways (Figure 1.1):

(i) Fail Safe Feature:

Each and every apparatus, component, circuit and sub-system employed

in a Signalling system shall be so designed that a failure occurring in any
of its component or sub-system shall result in a Safe-side Failure of the
system as a whole. This feature requires that any failure in a Signalling
system should trigger in displaying the most restrictive aspect on the
Signal (s). This is necessary to ensure safety.
(ii) One aspect one indication:

(a) Aspects of fixed Signals shall be nomenclatured to be

distinctive and unambiguous. This ensures Clarity of
Mind in the Driver. With this Feature, there should be no
possibility of one Aspect of a Signal being mistaken for the
other.

(b) One Aspect shall have the one name and one indication.
It should convey a distinct indication to the Driver.
Conversely for a given indication, the same Aspect shall be
used everywhere and at all times. It is of paramount
importance that the Aspect of a Signal communicates the
same intelligence to the Driver under all conditions. It will
be dangerous if the Aspect of a Signal conveys one
indication under certain set of conditions and a different
indication under certain other set of conditions. Intelligence
conveyed by the Aspect of the Signal should enable the
Driver to know explicitly what it means and what action is
called for on his part in controlling his Train.

(c) Action required by a Signal Indication shall be definite and

capable of easy implementation. This principle dictates
the location of a Signal and inter-Signal spacing and in a
way defines the point at which action on the said Aspect /
Indication should be commenced by the Driver to ensure
safety. This means that Signals should be so located with
respect to each other that an Indication from a Signal
displaying a restrictive Aspect, can be complied with by
Driver by means of normal brake applications initiated at
the foot of that Signal post itself and as a result either the
Train should come to a stop without the loss of speed at
the Signal in advance displaying Danger Aspect or the
speed of the train should get reduced to a speed which is
appropriate to pass any other Aspect conveyed by that
Signal.

(iii) Sighting Distance of a Signal to be Zero:

Location of a Signal should provide Sighting Distance (SD) required for

acting upon the indication conveyed by it. In the early stages of
development, it was thought that Signal Aspects should be visible over
long distance to enable the Driver to react on the Indication / Intelligence
conveyed by it but the Sighting Distance being an uncertain factor,
depending upon the environmental conditions, the thinking now has
changed and it is now considered necessary to design a Signalling
system and locate Signals in such positions so that Aspect / Indication
conveyed by the Signal provides sufficient time for action by the Driver
even if such an action is initiated at the footstep of the Signal post itself.
This means that Signalling system should be such that Driver can react to
the Aspect / Indication conveyed by the Signal even at the foot of that
 Signal and yet keep his Train under perfect control i.e. there should not
be any requirement for SD, which would mean that Sighting Distance of
a Signal could be zero. As a corollary, each Signal should pre-warn
about the Aspect of the Signal in advance. In this scheme of thinking,
there is, perhaps, no place for simple two Aspect Signalling which has
been the common Signalling adopted on Indian Railways till recent past.
Multi Aspect Colour Light Signalling (MACLS) meets this requirement fully
and action on the Aspect of the Signal can be even undertaken at the foot
of the Signal as each Signal is pre-warned for the Aspect of Signal in
advance.

(iv) Normal Danger System:

This principle requires one operation and one clearance of the Signal. In
this system, once the Signal is taken 'Off' for a particular movement and
the Train for which this signal was taken 'Off' moves past the Signal, the
Signal should revert back to its normal Aspect. The second
clearance for this Signal for a subsequent movement should necessarily
require normalising of all operations done for the preceding movement
e.g. if a Signal requires clearances from three different agencies like the
situation obtainable in end Cabin working, all the three agencies viz. the
two end Cabins and the third the controlling Station Master should be
prompted to normalise their controls after the completion of movement
and before a second move on the same Signal can be initiated by the
main operating agency of that Signal. As a corollary to this requirement,
each controlling agency of the Signal should be capable of withdrawing
back the control exercised by it to throw the Signal (s) to Danger position
in event of emergencies.

(v) Minimum number of fixed Signals on a Route:

Number of Signals in a Route is decided on considerations of safety and

requirements of Track capacity. Dictum of minimum number of Signals in
a Route enables smooth flow of traffic with minimum exertion and strain
to the Driver. Number of Signals, in fact, should, be on 'need to have'
basis.

(vi) Visibility of Signal Aspect from place of operation:

The Aspects displayed by Signals which are operated or controlled

manually should be visible from the place of operation or control and in
the case of Block Stations worked under Absolute Block system, also
from the place where the Block Instruments are located i.e. Station
Master’s Office.

In the case of Semaphore Signalling, Signal Arms which are not so

visible, by day, are electrically repeated in Cabins and SM’s Office, as
required. Semaphore Signals, front lights of which can not be seen by
night from place of operation or the place where Block Instruments are
located are provided with white Back Lights.
 In the case of Colour Light Signals of which the Aspects cannot be seen
from place of operation or control, are electrically repeated. Repeaters
take the form of miniature light units in case where Signals are operated
from Mechanical Lever Frames. In case of Colour Light Signals, operated
from Relay Interlocking, Repeaters are included on the Operating Panel
itself. In such cases even Route line up is also indicated on the Operating
Panel.

(vii) Continuous Visibility of Signal to the Driver:

Signals should be so located so as to be continuously visible to the Driver

from the time instant, the Aspect is picked up by him. Co-Acting Signals
are used for this purpose. In cases when it becomes impracticable to
locate a Signal suitably to provide continuous visibility to the Driver as
also to afford required Sighting Distance to him, Repeating Signals are
made use of to repeat the Aspect of Signal in advance.

(viii) Reliability, Availability, Maintainability and Serviceability

(RAMS):

(a) Specifications for Signalling component and sub-

systems for vital installations should be exacting to offer
high Reliability and Availability. Failures cause
disruption and serious delay to the movement of Trains
and failure situations, in addition, are prone to lead to
unsafe conditions.

(b) Signalling Systems should be easy and quick to

Maintain and Service without having to cause serious
interruptions to traffic.

(c) On a high traffic density route, systems of modular design,

which caters for modular growth and could afford easy
replacement at component / sub-system level, should
only be provided.

(d) It should be possible to record the log of events in a

system with a view to facilitating the monitoring and
Predictive Maintenance of Signalling gears.

(e) Adequate redundancies should be built in the system

for fallback option to stand-by mode of operation.

(ix) Block Signalling to be independent of Yard Signalling:

Signalling arrangement shall be planned and designed in such a manner

that Block Signalling shall remain independent of Yard Signalling to the
extent possible. This means that Block working should not be affected
by failures in Yard Signalling and the two should be insulated from
each other to the extent possible to provide flexibility in operation.
(x) Operation Friendly Signalling:

Design of Signalling and Interlocking system should cater for avoidance

of operational bottlenecks in the yard and should provide for possible
short shunts and simultaneous moves. Redundancies should be built
in the system to take care of vital failure modes to facilitate working
through stand-by modes until the failure is rectified and normalcy
restored.

(xi) Uniformity of Signalling on a section:

Uniformity of Signalling on a stretch of track is of paramount importance.

Viewed from Driver's point, it induces great confidence in him as he
correctly knows what is to come next and when. Uniformity of
Signalling on a section is also advantageous in Maintenance
Management and up-keep of spares inventory.

(xii) Overlaps:

Overlaps requirement for the system of Aspects chosen should be clearly

specified and provided for in the Signalling Plan. Overlap, as defined in
General Rules, is the distance to ensure safety. A Signal can display an
'Off' aspect only when the line is clear not only upto the Signal next in
advance but also up to a specified Overlap distance beyond it. Overlap is
a function of Aspects in a Signalling system and should reduce as the
number of Aspects provided in the Signalling scheme is increased.

In 2-aspect Signalling (Semaphore / Colour Light), the Block and Signal

Overlaps are respectively 400 & 180 metres. Overlaps are reduced to
180 & 120 metres respectively in MACLS / MAUQ and MLQ Signalling.
In Automatic Block, there is no Block Overlap and the Signal Overlap is
120 metres, as in MACLS / MAUQ Signalling.

1,4 DEFINITIONS OF TERMS USED IN RAILWAY SIGNALLING:

Station:

Means any place on a Line of Railway at which traffic is dealt with, or at which an
Authority to Proceed is given under the System of Working.

Stations where an ‘Authority to Proceed’ is needed to be granted to a Train are called

Block Stations.

Non-Block or Flag Stations are those where traffic (Passenger or Goods) is handled,
may be by way of booking but from where no ‘Authority to Proceed’ is needed to be
granted.

Block Cabins or Intermediated Block Huts (IBHs) also come under the generic
definition of the Station as ‘Authority to Proceed’ is necessarily given at these
Operating Points.
Station Limits:

Means the portion of a Railway which is under the Control of a Station Master and is
situated between the outermost Signals of the Station or as may be specified by Special
Instructions.

On Double Line, Station Limits will be separate for each direction.

Station Master:

Means the person on duty who is for the time being responsible for the working of the
traffic within the Station Limits and includes any person who is for the time being in
independent charge of the working of any Signals and responsible for the working of
Trains under the System of Working in force.

Important points to be noted in the definition are:

(i) Station Master is the Overall Incharge of the

Station and controls the Reception & Dispatch of Trains in to and out of
the Station. He may be assisted by Assistant Station Masters /
Cabinmen / Porters, depending upon the workload and types of operating
facilities provided at the Station.

(ii) Jurisdiction is within Station Limits.

System of Working:

Means the System adopted for the time being for the working of Trains on any portion of
a Railway.

On Indian Railways, mostly Absolute Block System and Automatic Block System of
working are in use.

Absolute Block System of working Trains is based on ‘Space Interval’ system wherein
a controlled rail section is divided into a number of smaller rail sections, each called a
‘Block Section’ and only one Train is allowed to run at a time in one Block Section. This
Space Interval between the two Trains i.e. entry into the Block Section is secured by the
human agencies in the form of two Station Masters, posted at the two end Stations of
the Block Section. Station Masters use Block Instruments to obtain permission to
approach prior to allowing a Train to enter in the Block Section.

In Absolute Block working, no train is allowed to enter the Block Section until the
permission to approach has been received from the Block Station in advance.

The essence of Absolute Block System lies in the fact that only one Train can remain in
one Block section at a time. On Double line sections, as Trains follow in the same
direction one after the other, there are no opposing Trains on the same rail section. On
Single lines, however, Trains are to be run on the same rail section from both the sides
and hence it is essential to ensure that when Trains are running in one direction in a
Block section, there is no Train allowed to run in the opposite direction.
Automatic Block System of working Trains is also based on ‘Space Interval’ system
wherein also a rail section is divided into a series of smaller sections, called ‘Automatic
Signaling Sections’. Contrary to the Absolute Block System, the Space Intervals in
Automatic Block System are secured automatically by the travel of a Train in to, through
and out of the Signalling sections. Each Signalling section is protected by a Stop Signal
at its entry point. The control mechanism to detect presence of a Vehicle / Train on a rail
section and thereby control the protecting Stop Signal is either through Track Circuits or
with the help of Axle Counters.

In Automatic Block System also there are semi-Automatic and Manual Signals, which
have an element of manual control like authorising of movement over Points or control of
traffic over Level Crossings etc.

General Rules of Indian Railways, in addition, cater for four more systems of Train
working i.e., ‘Following Trains System’, ‘Pilot Guard System’, ‘Train Staff & Ticket
System’ and ‘One Train only System’. In all the above Systems of Train working,
however, no Train can leave a Block Station, unless an ‘Authority to Proceed’ under
the System of working is given to the Driver.

In ‘One Train or One Engine only’ System, only one Train is allowed in the section at a
time. ‘Authority to Proceed’ is a ‘Token’ to the Driver. Safety of the System is based
on the fact that the same object (Token) cannot be in two places at the same time. The
System is applicable for short shuttle working of Trains on short Single line Branch line
sections such as Colliery and Wharf. Added safeguard is that only one Engine or 2 or
more Engines coupled together are allowed on the line, at one time.

‘Train Staff and Ticket’ System allows any number of following Trains to be sent in the
same direction. The Stations at both ends of the section have stock of specially printed
Tickets. When more than one Train is to pass through a section in the same direction,
the Drivers of all Trains except the last one are given Tickets as ‘Authority to proceed’
and the last Train takes the ‘Staff’ which is, in fact, the key of the Ticket Box and
functions as the ‘Direction Regulator’. When the ‘Staff’ is received at the Station at the
other end of the section, it can be utilised to unlock the Ticket Box and use the Tickets to
send the Trains from other direction. The ‘Staff’ is so interlocked with the Ticket box that
it cannot be taken out unless the Ticket box is locked. This ensures the safety in Block
operation.

Instead of ‘Staff’, a Railway man can be nominated to fulfill the same function i.e. either
to accompany the Train or give a ‘Ticket’ personally. This constitutes the ‘Pilot Guard’
System. Pilot is identified by his Dress and Badge that he wears.

In the ‘Following Train’ System, Trains are allowed to follow each other in quick
succession on the same line at prescribed interval of time. A ‘Time Interval’ is, thus,
maintained between the consecutive Trains. This System also requires granting of ‘Line
Clear’ from the Station in advance for the first Train to enter into the section.

Train:

Means an Engine with or without vehicle attached or any self-propelled vehicle with or
without a Trailer, which cannot be readily lifted off the track.
Block Section & Station Section:

Block Section means that portion of the Running Line between two Block Stations on to
which no Running Train may enter until Line Clear has been received from the Block
Station at the other end of the Block Section.

Sanctity of Block Section is very important in Train operations. Under the Absolute Block
System of working, only one Train can be allowed to remain in a Block Section at a time.
During the course of shunting, if Block section is to be necessarily infringed, operations
of ‘Block Back’ or ‘Block Forward’ are to be undergone prior to affecting such
infringement.

Figures 1.2 & 1.3 show Block Section limits on a Double Line and Single Line Stations,
equipped with Multi-Aspect Colour Light Signalling. It may be seen that Block Section
extends from Last stop Signal (LSS) of a Station up to First Stop Signal (FSS) of the
Station in advance plus an Adequate Distance, called the Block Overlap, beyond it.
Block Overlap in Multi-Aspect Signalling has been fixed at 180 ‘m’. It is 400 ‘m’ in 2-
Aspect Signalling.

Portion of the Station Limits, left out of Block Section is called the Station Section.
Station Sections are also shown in Figures 1.2 & 1.3. At Class ‘A’ Stations, there is
practically no Station Section and the Block Section extends up to Berthing Lines, as can
be seen from Figure 1.4.

Authority to Proceed:

Means the Authority given to the Driver of a Train, under the System of Working, to
enter the Block Section with his Train,

Line Clear:

Means the permission given from a Block Station to a Block Station in rear for a Train to
leave the latter and approach the former; or the permission obtained by a Block Station
from a Block Station in Advance for a Train to leave the former and proceed towards the
latter.

Facing and Trailing Points:

Points are Facing or Trailing in accordance with the direction a Train or vehicle moves
over them. Points are said to be Facing Points when by their Operation a Train
approaching them can be directly diverted from the line upon which it is running.

Level Crossing Gate:

Level Crossing means the intersection of Road with Railway track at the same level.

Level Crossing Gate means any form of movable Barrier, including a Chain capable of
being closed across the Road at the Level Crossing, but does not include a Wicket or a
Turnstile for the use of Pedestrians.

Interlocking:
Means an arrangement of Signals, Points and other Appliances, operated from a Panel
or Level Frame, so interconnected by Mechanical Locking or Electrical Locking or
Electronic Locking or any combination of these Lockings that their Operation must take
place in proper sequence to ensure Safety.

Points and Trap Indicators:

Are not Signals, but are Appliances fitted to and working with Points to indicate by day
and by night the position in which the Points and Trap Points are set.

Running Line:

Means the line governed by one or more Signals and includes connections, if any, used
by a Train when entering or leaving a Station or when passing through a Station or
between Stations.

Shunting:

Means the movement of a vehicle or vehicles with or without an Engine or of any Engine
or any other self-propelled Vehicle, for the purpose of attaching, detaching or transfer or
for any other purpose.

Main Line:

Means the Line ordinarily used for running Trains through and between Stations.

Direction of Traffic:

Direction of Traffic means:

(i) On a Double Line, the Direction for which the Line is signalled.

(ii) On a Single Line, the Direction for the time being established, under the
System of Working, to allow Trains to move in that Direction.

Adequate Distance:

Means the Distance sufficient to ensure Safety. Common term used for ‘Adequate
Distance’ in a Signalling System is called the ‘Overlap’.

GR defines the Overlap as the distance, sufficient to ensure Safety.

Two types of Overlaps are made use of in Absolute Block System - the Block Overlap &
the Signal Overlap.

The length of track in advance of a Stop Signal, which should be kept clear before the
Signal next in rear can be taken 'OFF’, is known as the Signal Overlap. In other words,
to take ‘OFF’ a Stop Signal, the portion of the track not only up to the next Stop Signal
but also for an Adequate Distance (called Signal Overlap) beyond it has to be kept clear
(Refer to Figure 1.5).

No Signal Overlap is necessary for clearing Calling-On and Shunt Signals, the logic
being that both the movements either over the Calling-On or over the Shunt Signal are
low speed movements and the Train / Vehicle starts from the Stop i.e. from zero speed.

The Overlap provided for the Last Stop Signal in Absolute Block Territories is greater
than for other Stop Signals and this is referred to as the Block Overlap. Block Overlap,
then, is the extra length of track in advance of the First Stop Signal of a next Block
Station, which should be kept clear before Line Clear can be granted to the Station in
rear (Refer to Figure 1.5).

Overlap distances are a function of the number of Aspects in the Signalling System,
Overlap decreasing as the number of Aspects increases. By increasing the number of
Aspects, the Overlaps can be reduced without sacrificing the Safety for the following
reasons:

(i) The speed of the Train can be continuously regulated by the Signals in
Multi Aspect Signalling.

(ii) Signal Aspects are repeated by the Signal or Signals in rear in a

rational manner i.e. Signals are pre-warned.

(iii) Safety becomes less and less dependant on the uncertain factor
of the Sighting Distance. In Multi Aspect Signalling, the Sighting Distance
virtually gets reduced to zero. The driver can react to the Aspect of the
Signal when being at its foot as NBD is made available to him between ‘Y’
and ‘R’ Aspects.

Block Overlap is:

(i) 400 metres in case of Two Aspect Lower Quadrant Signalling or Two
Aspect Colour Light Signalling, and

(ii) 180 metres in case of Multiple Aspect (MAUQ or MACL) or Modified

Lower Quadrant (MLQ) Signalling.

Signal Overlap is:

(i) 180 metres in case of Two Aspect Lower Quadrant Signalling or Two
Aspect Colour Light Signalling, and

(ii) 120 metres in case of Multiple Aspect (MAUQ or MACL) or Modified

Lower Quadrant (MLQ) Signalling.

Above Overlap distances have been arbitrarily fixed based on experience and do not
depend on speed, gauge or gradients.
In MAUQ / MACL Signalling, the First Stop Signal displaying the 'ON' Aspect is
approached at a cautious speed, being pre-warned. In 2-Aspect Signalling, however, it
is not so. It may be argued that the Block Overlap in 2-Aspect Signalling can be reduced
where a separate Warner is used and this would be correct provided the operation of
Warner is completely reliable. But, so long as Warner is operated by Single wire without
a Reverser, the irregular operation of the Warner cannot be ruled out. When the Warner
is 'OFF'. in an irregular manner, conditions then obtaining would be worse than in
2-Aspect Class ‘B’ working. Also the 2-Aspect Warner is by no means a Repeater of the
First Stop Signal but a Multi Aspect Distant is.

In MLQ Block Overlap is reduced because a Distant similar to MAUQ Distant is provided
and this will display ‘Caution’ when the First Stop Signal displays the ‘Stop’ Aspect. SO
for the Main Home is reduced because the Warner is located below it and the Aspects of
the two taken together correspond with the Aspects of a MAUQ Home. When the Starter
is 'ON', the Warner displays ‘Caution’. SO for Loop Homes is reduced because the
speed over turnouts is restricted to 15 Kmph and also the Distant is held at ‘Caution’.
Therefore, the Driver reduces the speed to the extent necessary, even before the Distant
is passed.

In Automatic Block Signalling, there is there is no Block Overlap. There is only Signal
Overlap, which is 120 metres in case of MACL Signalling. Under Approved Special
Instructions, this can be reduced to 90 metres on Suburban sections.

Three types of Overlap arrangements are used in Automatic Signalling system:

(i) Ordinary Overlap System.

(ii) Absolute Overlap System.
(iii) Full Block Overlap System.

In Ordinary Overlap System, each Signal is located at an Overlap distance (120 metres)
in rear of the Signalling section it controls. In Absolute Overlap System, separate short
Track circuits (120 metres) are used for the Overlaps. In Full Block Overlap System, the
Overlap is made equal to a Full Signalling section i.e. a Block.

Track Circuit:

Means an Electrical / Electronic Circuit provided to Detect the presence of a Vehicle

on a portion of track, the rails of the track forming part of the Circuit.

Axle Counters are also used to detect the presence of a Vehicle on a portion of track. It
is a Device, which when provided at two given points on the track, proves by counting
Axles in and Axles out whether the section of track between the said two points is Clear
or Occupied.

Isolation:

The ‘Isolation’ denotes the condition in which a line for a particular movement is
separated from all adjoining lines connected to it in such a manner that the Isolated Line
cannot be fouled or interfered with by any movement taking place on the adjoining lines.
Special Instructions:

Means Instructions issued from time to time by the Authorised Officer in respect to
particular cases or circumstances.

Fouling Mark:

Fouling Mark means the mark at which the infringement of fixed Standard Dimensions
occurs, where two Lines cross or join one another.

1.5 BLOCK SIGNALLING & STATION SIGNALLING:

1.5.1 Block Signalling:

Block Signalling is meant for Signalling a Train from one Block Station to the other. In
Block working, ‘Line Clear’ is required to be obtained from the Station in advance. To
enable this, it is necessary to have some means of Communication between the
adjacent Stations. This could be through:

(i) Communication Instruments.

(ii) Block Instruments:
(iii) Track Circuits or Axle Counters.

Where ‘Block Instruments’ are not provided, electrical Communication Instruments are
used for Block working and ‘Authority to Proceed’ is a ‘Line Clear Ticket’ or an
equivalent document, which is given to the Driver. Where ‘Block Instruments’ are
provided, every Train in its progress from Block Station to Block Station is required to be
Signalled in conjunction with these Instruments. A prescribed Code / Mode of working is
followed in carrying out the Block Operations.

On Single line, Trains are normally worked with ‘Token’ type Block Instruments. A
‘Token (Ball shaped) or Tablet (Disc shaped)’ is got released electrically from one of a
pair of interconnected Block Instruments with the co-operation of the Station Masters at
the two ends of the Block section. This ‘Token’ is the tangible Authority for the Driver of
a Train to take his Train into the Block section. On busy Single line sections, Tokenless
Block Instruments are used which simplify the Block operation, besides increasing the
Line Capacity.

On Double line sections, since Trains always run in the same direction on a line, Tokens
if used, would get accumulated at the advanced Station and arrangements would have
to be necessarily made to bring back and balance the Tokens frequently. If Token
balancing is not done in time, Block working might have to be suspended for want of
Tokens at a Station. Therefore, non-Token types Electric Block Instruments, called
Tokenless Block Instruments, are used on Double line sections.

1.5.2 Station Signalling:

Station Signalling is meant for Signalling a Train in to or out of the Station. Station
Signalling, inter-alia, includes provision of the Reception and Departure Signals and
Control on these Signals by the respective Operating as well as other controlling
Agencies. Station Signalling also provides for Shunt Signals which cater for short
Shunting movements within the Station section. Departure Signals at a Station are
invariably interlocked with the Block working mechanism provided for Signalling a Train
from one Block Station to the other.

One of the Cardinal principles of Signalling (stated in Para 1.3) is that Block Signalling
should be kept independent of the Station / Yard Signalling. This principle demands that
the Signalling arrangement shall be planned and designed in such a manner that Block
Signalling remains independent of the Station / Yard Signalling to the extent possible.
This would mean that Block working shall not be affected by the failures in Station / Yard
Signalling and the two shall be insulated from each other to the extent possible to
provide flexibility in Operation.

1.6 BLOCK SIGNALLING – TIME & SPACE INTERVAL METHODS:

Purpose of Block Signalling is to keep the Trains on a Railway section adequately

spaced apart to prevent Collision between the Trains running in the same direction and
prevent Trains entering on the same Track from the opposite direction.

Basically, two types of Controls are required to be catered for. If two separate Tracks are
provided for Trains running in opposite directions, then one Set of Control can be
provided to space the movement of Trains running in the same direction so that
adequate ‘Interval’ is available between two consecutive Trains. On the other hand, if a
single Track is used for movement of Trains in both directions, then another Set of
Control is required to prevent a Train in the opposite direction from coming on the same
Track when a Train is already occupying it.

1.6.1 Time Interval Method:

In the first case of spacing of Trains in the same direction, the Spacing should be such
that if a Train stops, then, the following Train Driver should be able to notice it and apply
Brakes to his Train so that it stops short of the preceding Train. The most important
Aspect is braking to a stop from the Speed at which a Train is running. 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 is the case when Trains
follow one another in quick succession. In actual practice, where Interval between Trains
is longer, a following Train does not see the earlier Train, and the Driver has to
continuously guess as to where the earlier Train will be. 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. This Time Lag should be such that the Train, which
has a stop, is able to reach the next stop within this Time. Thus by having a Time
Interval between Trains, a certain amount of Control can be achieved. But, in the case of
Railways, this is not much practicable, as:

(i) Different types of Trains like, Mail / Express, Passenger, High-speed

Freight and Low-peed Freight Shunting Trains are run.

(ii) The Speeds of all the Trains are not same.

(iii) The Terrain of the Country is not same everywhere.

(iv) The Brake Power, Hauling Capacity, Load of Train is not same for all
Trains, and

(v) 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 is called the ‘Space Interval Method’.

1.6.2 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.

This ‘Space Interval’ or ‘Block’ is controlled at the Entry. This Controlling Point should
know whether the Train, which had entered this Space, has vacated it so that another
following Train can be sent. Since the length of a Block is beyond the Normal Visual
Range, another Controlling Point is set at the end of the Block. This point can know
whether the Train, which had entered the Block, has arrived and advise so the
Controlling Point at the entry. Hence, with the two Controlling Points and
intercommunication between the two, it is possible to control the Entry of a Train into a
Block only when it is vacant.

The Information about the condition of this Block is given by the Exit Point to the Entry
Point, and the Entry Point transmits this Information to the Driver of a Train. Also the
Driver of the approaching Train must be able to know whether the next Block is not Clear
or not. Here is where the ‘Block Signalling’ comes in.

On Indian Railways, mostly the two Systems of Block Control are in use – Absolute
Block & Automatic Block. Both the Controls are based on ‘Space Interval’.

In Absolute Block System of working Trains, a controlled Rail section is divided into a
number of smaller Rail sections, each called a ‘Block Section’ and only one Train is
allowed to run at a time in one Block Section. This Space Interval between the two
Trains i.e. entry into the Block Section is secured by the human agencies in the form of
two Station Masters, posted at the two end Stations of the Block Section. Station
Masters use Block Instruments to obtain Permission to Approach prior to allowing a
Train to enter in the Block Section.

The essence of Absolute Block System lies in the fact that only one Train can remain in
one Block Section at a time. On Double Line sections, as Trains follow in the same
direction one after the other, there are no opposing Trains on the same Rail section. On
Single Lines, however, Trains are to be run on the same Rail section from both the sides
and hence it is essential to ensure that when Trains are running in one direction in a
Block Section, there is no Train allowed to run in the opposite direction.
In Automatic Block System of working Trains also, a Rail section is divided into a
series of smaller sections, called ‘Automatic Signaling Sections’. Contrary to the
Absolute Block System, the Space Intervals in Automatic Block System are secured
automatically by the travel of a Train in to, through and out of the Signalling Sections and
each Signalling Section is protected by a Stop Signal at its Entry Point.

1.7 PURPOSE & METHODS OF TRAIN DETECTION:

Process of Train Detection means to detect the presence or otherwise of a Train on a

portion of a Track. If there is a presence of a Train on the defined portion of the Track,
we say that the said Track section is occupied otherwise we infer that the Track section
under reference is Free or Vacant. The purpose of Train Detection Mechanism is to
ensure that second Train is not signalled on the portion of the Track, which is detected
occupied by another Train. Train Detection Devices, thus, assume significant importance
in maintaining the Safety in Train Operations. Binary Information conveyed by a Train
Detection Device is used for controlling the relevant Signal, which in turn controls the
Train Movements.

Methods of Train Detection Mechanisms fall under two distinct Categories – (i) Track
circuits, and (ii) Axle Counters.

A Track circuit is an Electrical circuit of which the running Rail of a Track forms a part.
Track circuit essentially consists of an insulated section of running Rails into one end of
which is fed a Source of Electrical energy and a Relay is connected at the other end to
complete the Electrical circuit. According to the nature of Supply Source, the Track
circuits are categorised as DC Track circuits, AC Track circuits and Electronic Track
circuits.

In the case of a Track circuit, the two Rails of the Track are required to be insulated
from each other. Hence an essential requirement for setting up of a Track circuit is the
use of Wooden or Concrete Sleepers. A Track circuit can not be configured on a Track,
equipped with Steel Sleepers.

In the case of Axle Counters, the numbers of Axles entering a specified portion of Track
at one end and leaving at the other are counted Electronically and if the ‘In-count’ and
the ‘Out-count’ tally, the section is deemed to be clear of the presence of Vehicles. A
pair of Axle Counters is installed at either end of the Track and as a Train enters the
Track section from one end, the number of Axles entering the section are electronically
counted. Similarly, when the Train leaves the Track section at the other end, the Axles
are counted again at that end. If the same number of Axles are counted at both the
ends, it indicates that the complete Train has vacated the section and the section is free.
In case of unequal Counts, the section is declared as still occupied.

Both the Track circuits as well as Axle Counters are designed to be Fail-safe Devices
and any failure of their Components or connecting Accessories creates situations of an
occupied section, even though the section may be actually free from Vehicles.

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