Basic Railway Signaling

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Railway Signaling

Basic Principles of RailwaySignaling

Ir. Linus Andor Mulana SijabatTechnology and peration!irectorPT Len Railway Syste"s


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Introduction When trains run on railway tracks they follow rules of operations in which safety plays a very important role The most important rule in respect of safety is to ensure that two trains do not occupy the same position on

the track at the same time

To make this rule work operation of trains uses signaling to control movement of trains on tracks and dividestracks into several sections which are protected by the signals

TR1

OUT = !T1

"1

TR#

"#

TR$

"$

T! T# T$

%ig 1&1

The hori'ontal liner represents the railway track( the signals are depicted by the symbol ofthe circle with a hori'ontal and vertical line to this circle and the red rectangles are thetrains

This representation is however to e)plain how trains are run safely and the actualrepresentation of various functions are e)plained later

"afety of train running is ensured by allowing only one train to enter any track sections at a

time The entry to the track sections are controlled by signals placed short of the boundary of

%ig 1 shows a

representation of a railwaysignaling arrangement The signals depicted are

named "1( "#( "$ etc The railway track in the

above e)ample is dividedinto + sections called T!(

T1( T# and T$ Trains are referred to as

TR1( TR# etc


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In addition to straight sections as shown above when train movements take place on

diversions safety of such movements are taken care of by signaling system , point is a portion of the track which is a potential danger and can cause trains to be

derailed if not properly controlled

TR1

T1

"1TR#

"$

T! T#

%ig 1

"+

"#

-1

T$

%igure 1 depicts thescenario around a diversion

The diversion is created onthe track by what is referred

to as point -1 is a point on the track and

allows a train approaching itto be diverted to the left orallowed to go straight

In %ig 1 signals "#( "$ and"+ allow this control overtrains approaching thediversion -1&

The signaling system ensures that this is controlled properly To ensure this signals are provided controlling trains moving towards the point and

unless the point is detected to be safe for the intended movement the signals leadingto it will be held at red

When a train is approaching a point as TR# approaching point -1 the point has to be inthe correct position and also must not be liable to move under the wheels of the train

The correct position of the point is important because .uite often the speed allowedwhen a train takes a diverging route is re.uired to be restricted so that the train do not

derail while negotiating the diversion Therefore the position of the point needs to be detected and this is referred to as the

lie of the oint


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, point has a pivot about which rails swing to create either a straight route or adiverging route

/ue to this arrangement there is a possibility that the rails forming the point can

swing under the wheels when a train is moving on it

To ensure this a point is locked in the straight route or in the diverging route bysuitable means

The signaling system detects that the lock is in place before clearing a signal andallow a train following the signal to move over the point


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Loc#ing

There are three types of locking /irect

,pproach

Route

TR1T1

"1

TR#

"

T!T# T$

"+ "#

-1

"0 T

-#

"$

T+

%ig 1&$


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!irect Loc#ing

This is the most fundamental level of locking /irect locking is available as long as a signal is clear or track is occupied or a point is set

When signal "1 is cleared the cleared condition of the signal locks other signals which cancause trains to run on any part of the route over which "1 allows a train to run

Thus with "1 cleared allowing trains to move to track T1 signal "+ cannot be cleared and willbe locked as the latter also allows trains to occupy track T1

Other form of direct locking is the locking of the point in the route for which signal "1 iscleared

If "1 is cleared to the straight route T! T1 T# then the point -1 will be set and locked toallow a train to move on the straight route over -oint -1

,ttempts to move point -1 from this position will not be allowed and hence will be locked

2onversly if the point -1 is not set for straight the signal "1 will be locked

The occupation of a track also locks signals if T1 is occupied then signal "1 cannot be cleared

"ignal "1 is therefore directly locked to the cleared status of the track

-oints are directly locked to track circuits over the point 'ones

If the track circuit over a point 'one is occupied the it is locked so that it cannot move

This is the direct locking of the point


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Approach Loc#ing

While ensuring safety for train running it is not only necessary to ensure that safety is

ensured over all portions of track for which signals have been given but also over portions

over track which can get occupied due to trains approaching a signal which protects these

portions of track failing to stop at this signal

"uch protection is re.uired under two conditions

When the signal protecting had not been cleared

When the signal protecting had been cleared at some time but had been subse.uently put back to

danger

This feature is referred to as approach locking of a signal Once a signal has been cleared it is sub*ected to approach locking meaning that even if

this signal is put back to red the route over which this signal would have allowed trains to

go will not be available for other routes unless it is established that the train that was

approaching the signal will not have reasons to go past the signal

%or this it is assumed that if a train has come to a stop it will not go past a red signal but if

it is moving it man not be able to stop right at the foot of the signal

The reason why a moving train may not be able to stop before a signal can be due to the

following reasons3a4 The signal was put back to red at the control but before the command was processed and the signal

actually indicated red the train may have gone past the signal and so did not stop

b4 The train had been very close to the signal when the signal turned red and so the train could not bestopped before the signal

c4 /ue to reasons like a failed brake system or a inattentive control of train the signal was overshot

,pproach locking provides protection against a4 and b4& c4 cannot be protected against

e)cept by use of other forms of signaling as ,utomatic train protection system


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2onsider that due to change of mind the train controller decides to stop the train TR1 at signal "1 andmove train TR# to move onto track T1 and through diverging setting of point -# onto track T+

To do this the signal "1 is put back to danger and then signal "+ is attempted to be cleared

5ecause of the delays involved in the carrying out the commands thought and the indication given by

the signals at site it is possible that the train TR1 passes the signal "1 before it had turned red( lands on

track T1 and starts to progress further not aware that the signal "1 it had passed was intended to be putback to red and TR1 stopped before it crossed

When a signal is signal is cleared if there are points in the route then the points are kept locked in theposition re.uired for the route

,s long as the signal is cleared this locking is maintained directly by the signal which has been cleared

6owever( if the signal is replaced to red and when a train is approaching it unsafe conditions can occuras e)plained above

To protect against this approach locking is used to lock the point in the desired position

TR1T1

"1

TR#"

T! T# T$

"+ "#

-1

"0 T

-#

"$

T+

%ig 1&+

The importance approach locking

is e)plained in 7g 1&$ 2onsider that two trains TR1 and

TR# apporaching each other 2onsider initially signal "1 had

been cleared to allow train TR1 tomove towards the track T18 Ttaking divergence at point -1( Thetrain TR# was to wait till train TR1

goes past the track T1 and ontotrack T


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Route Loc#ing

When a signal say , is cleared to allow a train to move past its safety is checked by thesignal to the ne)t signal in the route of the signal say 5

When a train passes a signal the signal is replaced back to red

9ven though the signal is put back to red the train is continuing its *ourney till it reachesthe ne)t signal

It is necessary to ensure that during this part of the movement of the train it is protectedagainst trains which can come from the opposite direction

There is also need to ensure that any point in the route remains locked in the position it isdesired for the route until the train has reached the destination signal and come to a stop

%ig 1& shows a condition when a train TR1 is planned to be sent to track T+ from track T!

TR1

T1

"1

T! T#

%ig 1&

"+

"#

-1

T+TR#

T

T$

When signal "$ is cleared it ensuresthat the point -1 cannot be moved

6owever once train TR1 goes pastsignal "1 onto track T1 the signal "1 isput back to red train TR1 isapproaching the point -1 and track T$

Until the train moves to track T it hasto be ensured that point -1 is keptlocked towards T

It is also re.uired to be ensured thatthe train TR# is not at the same timeapproaching towards track T$

The locking of the point -1 and thetrain TR# under this condition is


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$lan# protection and isolation

When a train is allowed to move by a signal it is also necessary to ensure that no part ofthe train will be involved in a side collision

When train TR1 moves to past signal "1 if there is a vehicle on track T: in the divergingportion there is a possibility of this vehicle colliding with the train TR1

To provide protection against this the end -#5 needs to be kept in a position so that anyvehicle occupying T: cannot cause obstruction to a train travelling straight on the -#, end

This is known as ;ank protection or isolation and needs to be ensured

TR1

T1

"1

T! T#

%ig 1&

"+

"#

-1

T+TR#

T

T$

TR$

T0 T< T

"$

T:

-#,

-#5


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Protection in the o%erlap

When a train is approaching a signal a possibility e)ists that the train may fail to stop atthe signal where it is intended to stop due to mechanical failure or due to human failure

While there is no absolute arrangement to control against this eventuality a partialsafety is ensured by providing a small part of the track beyond the signal at which the

train is to stop free of any con;ict or obstruction to the train if it fails to stop at the footof the signal

Typically when train TR1 is approaching "1 it will normally be ensured that the tracksection onto T# is free of any obstruction

This includes possibility of any train from the opposite direction reaching T#

6ence if TR1 is allowed to approach "1 it will be ensured that the train TR# does not atthe same time approach signal "#

TR1

T1

"1

T! T#

%ig 1&0

"+

"#

-1

T+TR#

T

T$

TR$

T0 T< T

"$

T:

-#,

-#5

,ny point in this portion of the track alsoneeds to be set and locked in theposition allowing safe movementthrough it

If TR1 is approaching signal "1 it willmean point -#, must be set and lockedfor the straight route

The point -#, and track T# is referred tobe in the overlap for signal "1 and locksthe signal allowing approach of a train tosignal "1 if not found free

2onversely if signal "1 is cleared any

condition which can lead to the overlapfrom failing to remain in the condition tomaintain safet for train TR1


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Release of loc#ing

"ignals indicate when a route which it checks is safe for a train to travel The safety is checked from di>erent angles as e)plained above

,fter a signal has been cleared for a train it is re.uired to be put back to danger as the train movespast it

There are two reasons for doing this To ensure the safety of the train which has moved past

To allow clearance of other signals which has been locked by it

The release of locking is done automatically as a train moves along the route a signal had cleared for it

The locking is released in stages ,s the train moves past the signal the approach locking is brought back to normal

,s the train clears the 7rst track after the signal the direct locking gets released

,s the train moves the route locking ( ;ank or isolation protection for the portions of the route cleared by the trainis removed

,fter the train has come to a stop at the ne)t signal for su?cient time to prove that it is not moving the overlap isreleased

This is the normal release with the passage of train

There can however be occasions when it is re.uired to cancel a signal which has been cleared and yet

to be passed by the train

When this is re.uired the signal is canceled

When the signal is canceled it is necessary to ensure that the locking it had enabled also get canceled

6ere again the cancellation starts from release of the approach locking followed by release of the route

locking( locking of ;ank and isolation and 7nally the overlap

The release is done only after it is established that a train which had been approaching the train has

come to a stopped at the signal before the locking to other signals are released


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Basic Railway Signaling

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