Poly unit 4.4

Transportation Engineering- C. Coomarasamy

Formerly Professor / Director, JRPC Trichy.

4.4 Signaling:Objects of signaling – Types – Classification according to location – Special signals – Typical layouts – Control of movement of trains – Following train system – Absolute block system- Automatic signaling – Pilot guard system – Centralised traffic control system.

4.4 Signaling

4.4.1. Objects of signaling4.4.2. Types4.4.3. Classification according to location 4.4.4. Special signals 4.4.5. Typical layouts 4.4.6. Control of movement of trains (a) Following train system (b) Absolute block system(c) Automatic signaling (d) Pilot guard system (e) Centralized traffic control system.

4.4 Signaling

Definition:Signaling consists of the systems, devices and means by

which trains are operated efficiently and tracks are used to maximum extent, maintaining the safety of the passengers, the staff and the rolling stock.

It includes the use and working of signals, points, block instruments and other equipments.

Objects of Signaling:1. To provide facilities for the efficient moving of trains.2. To ensure safety between two or more trains which

cross or approach each other's path.3. To provide facilities for the maximum utility of the

track.4. To provide facilities for safe and efficient shunting

operations.5. To guide the trains movement during maintenance and

the repairs of the track.6. To safeguard the trains at converging junctions and

give directional indications of diverging junctions.

4.4.1. Objects of signaling

4.4 Signaling

• Railway signaling is a system used to control railway traffic safely, essentially to prevent trains from colliding.

• Being guided by fixed rails, trains are uniquely susceptible to collision; furthermore, trains cannot stop quickly, and frequently operate at speeds that do not enable them to stop within sighting distance of the driver.

• The set of rules and the physical equipment used to accomplish this determine what is known as the method of working (UK), method of operation (US) or safe working (Aus.).

• Not all these methods require the use of physical signals and some systems are specific to single track railways.

• The earliest rail cars were first hauled by horses or mules. A mounted flagman on a horse preceded some early trains.

• Hand and arm signals were used to direct the “train drivers”.

• Foggy and poor-visibility conditions gave rise to flags and lanterns. Wayside signaling dates back as far as 1832, and used elevated flags or balls that could be seen from afar.

4.4.1. Objects of signaling

4.4 Signaling

A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers.

The driver interprets the signal's indication and acts accordingly.

Typically, a signal might inform the driver of the speed at which the train may safely proceed or it may instruct the driver to stop.

Signals are used to indicate one or more of the following:• that the line ahead is clear (free of any obstruction) or

blocked.• that the driver has permission to proceed.• that points (also called switch or turnout in the US) are

set correctly.• which way points are set.

4.4.1. Objects of signaling

4.4 Signaling

• the speed the train may travel.• the state of the next signal.• that the train orders are to be picked up by the crew.Signals can be placed:• at the start of a section of track.• on the approach to a movable item of infrastructure,

such as points/switches or a swingbridge.• in advance of other signals.• on the approach to a level crossing.• at a switch or turnout.• ahead of platforms or other places that trains are likely

to be stopped.• at train order stations.

4.4.1. Objects of signaling

4.4 Signaling

'Running lines' are usually continuously signalled.

Each line of a double track railway is normally signalled in one direction only, with all signals facing the same direction on either line.

Where bi-directional' signalling is installed, signals face in both directions on both tracks (sometimes known as 'reversible working' where lines are not normally used for bi-directional working).

Signals are generally not provided for controlling movements within sidings or yard areas.

4.4.1. Objects of signaling

4.4 Signaling

.

4.4.2. Types

Types of signals

Operatingcharacteristics

Functionalcharacteristics

Locationcharacteristics

Specialcharacteristics

can be classified into various categories based on

1.Detonating signals(Fog or audible signals)2.Hand signals(Visual indication signals)3.Fixed signals(Visual indication signals)

1.Stop or Semaphore signals2.Warner signals

3.Shunting signals

4.Coloured light signals

1.Reception signals (a) Outer signals (b) Home signals2.Departure signals (a) Starter signals (b) Advance starter signals

1.Repeater orCo-acting signals2.Routing signals3.Calling on signals4.Point indicators5.Modified lower quadrant Semaphore signals6.Miscellaneous signals

4.4 Signaling

1.Detonating signals(Fog or audible signals)2.Hand signals (Visual indication signals) 2.Hand

signals 3.Fixed signals(Visual indication signals) usually semaphore

type, fixed at a place . ( see next slide)

4.4.2. Types

Operatingcharacteristics

-fixed to rails by means of clips-explode with a loud sound-for safety placed at least 400 to 500 m ahead of signals

DANGER-STOP

CAUTION-MOVE SLOWLY

CLEAR-PROCEED.

in the case of a person riding in or on a vehicle, either arm waved up and down denotes STOP.

1.Detonating signals

During day time-by flags fixed to a wooden handle-or by bare armsDuring night time-by lamps movable glass slidesgreen- Proceedred – Stop and yellow- Proceed cautiouslyUsed by guards,Station masters,Cabin man,Gang man,Key man,Point man,or authorised man

4.4 Signaling

1.Stop or Semaphore signals (US - home signal or absolute signal)

2.Warner signals3.Shunting signals4.Coloured light signals

4.4.2. Types British semaphore stop signal (lower quadrant type)

Functionalcharacteristics

Stop Signal – Clear

Stop Signal – Danger

Stop Signal Post

Semaphore signals :-design is to show the stop position of any failure that happens to be in the apparatus. -in normal position- it indicates the-stop position.-placed on L.H.S of the direction of the movement-and the movable arm projects towards -the track for indicating the signal.-Semaphore: The semaphore arm of a -stop signal is red in front with a -white stripe near the end, and -white in the back with a -black stripe near the end. -The arm is square-ended. -Signal aspects are as shown.

Ordinary position-horizontal- ie.,ON position-indicates Stop or Danger-when the signal is pulled- 45 to 60 deg.-OFF-position indicates Proceed.

4.4 Signaling

A stop signal (US - home signal or absolute signal) is any signal --most restrictive indication is 'danger' (which compels a stop).

Stop signals are used to protect junctions, points (US - 'switches'), level crossings, movable bridges, platforms or block sections.

A particular signal box may control one or more stop signals on each running line. In a traditional mechanically signalled area, have two stop signals governing each line.

The first reached by a train is known as the home signal. The last stop signal, known as the starting or section signal, is

usually located past the points etc. and controls entry to the block section ahead.

The distance between the home and starting signals is usually quite short (typically a few hundred yards), and allows a train to wait for the section ahead of it to clear without blocking the line all the way back to the previous stop signal.

• At some locations, more than one home signal might be provided on the same line.

• These will be identified by names such as outer home and inner home, or first home, second home etc.

4.4.2. Types

4.4 Signaling4.4.2. Types1.Stop or Semaphore signals

2.Warner signals

Semaphore Warner Signal on a post by itself ('on'). Semaphore Warner Signal

on a post by itself ('off').

4.4 Signaling

Semaphore Warner signal in Two--

2.Warner signals: V notch at free end, and the white band is also V shape. A semaphore signalat the entrance to Station is combinedwith a warner signal.

4.4.2. Types

4.Colour-light: The stop signal may have two (green above red), three (green-yellow-red), or four lamps (yellow-green-yellow-red) . Aspects are as shown below.

4.4 Signaling

Ground Shunt Signal - Danger

Ground Shunt Signal - Clear

Ground Shunt Signal - Danger

Ground Shunt Signal - Clear

3.Shunting signals 4.4.2. Types

Type - shall be either – (a) a Disc signal or (b)a Position light signal . under special instructions ,a Shunt signal may be a miniature semaphore arm . Disc Signals -The front of the disc shall be white with a red bar .The back of the disc shall be white with a black bar .By day, the bar of the disc shall be horizontal in the 'ON' position and 45 to 60 deg. below the horizontal in the 'OFF' position in anti-clockwise direction .By night the signal shall display red light in the 'ON ' position and a green light in the 'OFF' position . Miniature Semaphore Signal-( a ) The semaphore arm of the Shunt signal shall be square ended. The front of the arm shall be red with a white bar .The back of the arm, shall be white with a black bar. The bars shall be parallel to the end of the arm.

4.4 Signaling

Location ( a ) Shunt signal may be separately located on the post

or close to ground or may be fixed below a stop signal other than the first stop signal of the station .

(b) Where shunt signal is required to be fitted on a signal post on which a Calling-on signal is also fitted ,the shunt signal shall be fitted below the Calling-on signal.

(c)A shunt signal; placed below a stop signal or a Calling-on signal shall not be capable of being worked at the same time as the relative stop signal or a Calling-on signal.

(d) Where a shunt signal is fixed below a stop signal it shall show no light in the 'ON' position .

Diverging routes -More than one shunt signal may be placed on the same post and when so placed ,the top most shunt signal shall apply to the extreme left hand line and the second shunt signal from the top shall apply to the next line from the left and so on .One shunt signal with or without route indicator may also be provided for a number of diverging routes.

4.4.2. Types

4.4 Signaling

1.Reception signals; are the signals which control the reception of trains into a station

(a) Outer signals (b) Home signals. At some stations, include Routing signals.

(a)Outer signals -is the first “stop” signal at a station, i.e., - indicates the entry of the train from block to station

yard - should be placed at adequate distance from the

point up to which the line can be obstructed after permission to

approach has already been given. Adequate distance: In India, 0.54 km for B.G, 0.40 km for

M.Gi.e., signals- provided at this distance beyond the station

limit.-generally one arm.-but may have warner signal on the same post nearly 2m

below it.-The driver must bring his train to a stop at a distance of 90

m before the outer signal and then proceed to the Home signal with caution.

4.4.3. Classification according to location Location

characteristics

4.4 Signaling

(b) Home signals:-Due to its location at door of station, it is termed as home

signal.-has bracketted arms to indicate which line is to be used.-it is one of the important functions of interlocking to

ensure that the route is properly set before the signal arm is taken to “OFF’ position.

-So the function of Home signal is to protect the sidings already occupied.

-is, therefore, located not more than 180 m from the start of the points or switches and unless there are other means of holding the route.

-lock retaining bars are provided to hold the route when the distance is more than 180 m .

-hence signals carry as many arms as the number of diverging lines.

4.4.3. Classification according to location

4.4 Signaling

If, it is in ‘Proceed position’ then the driver can take the train at normal speed , assuming that Home signal is also in the proceed position.

Two Aspect Signals- Outer Signal. : 1200 m in section where sectional speed is 100 km/ hour and

above. 800 m where sectional speed is less than 100 km/ hour. Where minimum visibility as above can not be achieved, warner way be separated. With the warner separated ,minimum visibility of outer shall

be not less than 400 meters. Warner Signal on a post by itself        400 m Home Signal                                         400  m Main Starter Signals                            400  m All other Signals                                    200  m Note : Where adequate visibility of stop signals cannot be

provided, repeater or co-acting signals shall be provided to ensure the combined visibility or speed restrictions imposed.

4.4.3. Classification according to location

4.4 Signaling

2.Departure signals are the signals which control the despatch of trains from the stations . Types: (a) Starter signals

(b) Advance starter signals

(a) Starter signals: - marks the limit up to which trains stopping at a

station come to a stand or halt.- Home signals carry as many arms as the number of

diverging lines whereas separate starting signal is provided for each line.

- The starter is the last stop signal at a station.- If advance starter is not provided then is such cases, it

has to be placed outside all connections of the line to which it refers unless permitted by approved instructions.

4.4.3. Classification according to location

4.4 Signaling

- Thus, starter signal actually controls the movements of the trains when they depart from the stations.

- No train can leave the station unless the starter signal shows the “Proceed” position (i.e., inclined position)

(b) Advance starter signals:-besides starter signal, an advance starter may also be

provided (generally 180 m or more) beyond the trailing points or switches.

-becomes the LAST STOP SIGNAL at the station where it is provided.

-in general, it is an indication for the train having left the station and

-is no more the responsibility of the station master.

4.4.3. Classification according to location

4.4 Signaling

.

Advancestarter

Advancestarter

UP movement

DOWN movement

640 m

Warner signal

Warner signal

Outer signal

Outer signal

185 m 185 m

Typical signal box layout

4.4.5. Typical layouts

Positions of signals in Station-yard

4.4 Signaling

• An advanced starting signal might be used at a location where it might be desirable to advance a train from a station platform before the section ahead becomes available.

• In this scenario, the starting signal permits the train to draw forward from the station area toward the advanced starting signal, which controls entry to the section ahead.

• If any of the signals beyond the first stop signal are at 'danger', the previous signals will also be held at 'danger' until the train is almost at a stand, to indicate to the driver that the next signal is at 'danger'.

• This can be enforced by instruction or by electrical interlocking, which requires the provision of a track circuit on the approach to the signal.

• In America, where the term home signal is in common usage, it generally refers to the "generic"

• British definition of 'stop signal', namely any signal whose most restrictive indication is 'danger'.

4.4.3. Classification according to location

A Junction Signal

Approach, Route and Track Locking

Signalling at a Double Track Junction

Interlocking. In this case, the interlocking could allow Points C4 to be set for the main line route and permit Signal C3 to how a proceed aspect

4.4 Signaling

1.Repeater or Co-acting signals 2.Routing signals 3.Calling on signals 4.Point indicators 5.Modified lower quadrant Semaphore signals 6.Miscellaneous signals 1.Repeater or Co-acting signals Generally, when a train passes through a station

without stopping, the driver comes across the five signals in this sequence.

WARNER,OUTER,HOME,STARTER andADVANCE STARTER.

Specialcharacteristics 4.4.4. Special signals

4.4 Signaling

1.Repeater or Co-acting signalsBut when the driver’s vision is obstructed by (i) an over bridge between the signals , or, (ii) sharp curvature in the alignment or (iii) due to any other reason, a signal is provided with a duplicate arm of smaller size at a suitable position which repeats the indication of the signal ahead. It is therefore, termed as a Repeater Signal. This signal is linked with the main

signal and, therefore, when the lever is pulled both signals are lowered simultaneously.

As these signals work in unison with the main signal, it is called “Co-acting Signal”.

Specialcharacteristics

4.4.4. Special signals

4.4 Signaling

Banner Repeater Signal ('on').

Banner Repeater Signal ('off').

Semaphore Repeater Signal ('on').

Semaphore Repeater Signal ('off').

Specialcharacteristics

1.Repeater or Co-acting signals

4.4.4. Special signals

4.4 Signaling

2.Routing signals:At big stations, where a number of lines exist at the station taking off different routes from the main line, besides one main Home Signal, Routing signals are provided at the points of diversion.Lowering of anyone of the signals indicates the track for which the points are set.The signal for main track (Home signal) is at a higher level while signals for other diversions are at a lower level.Number of the platform or line- indicated by a number

plate-e.g., 1,2, 3 etc.

Specialcharacteristics

4.4.4. Special signals

4.4 Signaling

Semaphore Calling-on Signal ('on').

Lower Quadrant Calling-on Signal ('off').

Upper Quadrant Calling-on Signal ('off').

Specialcharacteristics

3.Calling on signals:-Consist of small arms below and parallel to Home Signal fixed on the same vertical post.-When a train stops with Home signal indication, then a Calling on signal permits a train to proceed cautiously by keeping this signal arm at “PROCEED” position.- useful when repair works are going on.

4.4.4. Special signals

4.4 Signaling

4.Point indicators-mainly used to indicate the route to driver as a precaution against bursting of points or running into occupied line when the signals are not available for this purpose.-do not indicate the correct setting of points,-they should not be relied upon by the station masters as a mean of verification of the setting of points for movements in the facing direction.-consists of a rotating lamp-about its central vertical axis.-Lamp is enclosed within a box (four sides).- points are set for the main track-white target-Points are changed- rotates 90deg. green target - (green disc or green light at night)

Specialcharacteristics

4.4.4. Special signals

4.4 Signaling

In rare circumstances, one can find multiple signals placed on the same mast one above the other; in such a case, the convention is that the highest one refers to the leftmost divergence, and successive signals below it refer to successive routes to the right.Stop Signal – Clear

Railway semaphore signal

4.4.4. Special signals

4.4 SignalingSpecial

characteristics

5.Modified lower quadrant Semaphore signals- A distant signal placed at an adequate distance from the

Home signal.- A Home and Warner signal combination is placed at an

adequate distance (185 m from the point)

4.4.4. Special signals

Three-position Distant Signal showing 'Attention'

Three-position Distant Signal showing 'Proceed'

Distant Signal – Caution

Distant Signal – Clear

Distant Signal Post

.

Combined signals: When owing  to their location, it is necessary to combine two signals, one stop signal only may be provided under approved special instructions, capable of displaying any or all of the following aspects:-

(a) Danger. (b) Caution. (c) Attention. (d ) Clear.

Section Ahead Caution Section Ahead Clear

Combined stop and distant signalsBritish semaphore stop and distant signals on common post (lower quadrant

type)Where signals are closely spaced, a stop signal and a distant signal can be

mounted on the same post. The distant signal is always the lower of the two. The two signals are "slotted" so that the distant signal can only clear if the stop signal is clear. Both signals display a light at night, which means that the 'danger' indication appears as red over yellow.

Shunting and subsidiary signalsShunting signals and subsidiary signals also exist in semaphore form, with

smaller arms and lights than are provided for main signals. These signals may also take the form of a disc with a horizontal stripe which

is rotated 45 degrees to the clear position.Train order signalsIt was common for train order signals to point the arm straight down to

indicate 'proceed'.Non-railway usesSignals on the River Weaver Navigation, directing boats into the paired locksRailway-style semaphore signals have been used to control movements of

boats or ships (e.g. at swing bridges) and also to control road traffic (e.g. at level crossings).

Sighting Board

Subsidiary Signal Danger

Subsidiary Signal Clear

Subsidiary Signal Calling On

Subsidiary Signal Shunt Ahead

Subsidiary Signal Warning

Subsidiary Signals

A closer view of the down inner home, as a train enters the station. In the distance the double disc that controls the exit from the carriage sidings can be seen.

Diverging Junction Signal Lesser Route Diverging Junction Signal Main Route

This is the view in the opposite direction. The up advanced starter is cleared for a departing train, and on the right is the down outer home. It is unusual for two successive signals to have subsidiaries, but the one under the outer home is a warning signal which can be cleared either if the incoming train is to be called on at the inner home, or to allow the train to approach the latter if another train is leaving the station at the same time

Semaphore: The semaphore arm of a stop signal is red in front with a white stripe near the end, and white in the back with a black stripe near the end. The arm is square-ended. Signal aspects are as shown below. Colour-light: The stop signal may have two (green above red), three (green-yellow-red), or four lamps (yellow-green-yellow-red) as described above. Aspects are as shown below. 

Screen Shot of Signalling Layout Display in Control Room

Route Selection

Route occupation display

• .Tracking the movement of the train

The train has released the points

Two trains are displayed

A second route is selected

The third train arrives

Train 6460 moves towards the siding

The third route is set up

Two routes freed for Up and Down trains to proceed

4.4 Signaling4.4.6. Control of movement of trains

(a) Following train system (b) Absolute block system:

or (Space Interval system) or (Lock and Block system)

(c) Automatic signaling (d) Pilot guard system (e) Centralized traffic control system.

4.4 Signaling

One of the basic objects of signaling is to control the movements of trains with a view to ensure safety by preventing accidents. To achieve this goal , a particular system of working should

be adopted keeping in view three aspects.• Safety• Speed and • Traffic densityThe absolute block system is extensively used on the IR. In the block system, the railway track is generally divided

into block sections.A block system- distance between the last stop signal of

one block and the first stop signal of the block next in advance.

Automatic block system- distance between consecutive stop signals.

4.4.6. Control of movement of trains

4.4 Signaling

Various systems that work on one or the other railway for the control of movements of trains are the following:

-“One engine only” system. Initially only one engine-available-collision did not arise-

one engine –on the line-at a time. -suitable for small sections where shuttle trains work with

one engine and one rake.(a) “Following train” or “Train Interval” system:• In this system, trains are worked between two stations by

dispatching them one after the other (all in the same direction)

• at specific intervals (generally at least 15 minutes). • The trains are run at specific speeds (less than

25km/h). • This ensures that there should only be one train in every

5km stretch in the section between the stations.

4.4.6. Control of movement of trains

4.4 Signaling

• The maximum number of trains simultaneously present in the section is restricted to 4.

• Trains are dispatched only after the station masters of both stations have been in communication and have agreed upon the number and timing of the trains.

• The driver of a train being dispatched in this system must carry written authorization (Authority to Proceed, Following Trains System) specifying the destination, speed, and details of preceding and following trains on the section.

• This system was introduced in many areas as an emergency measure in 1941 in order to cater to the urgent wartime needs of the railways in India.

• It is still used in some areas when there is a requirement for large unidirectional movements of many freight trains (also known as the Corridor System).

4.4.6. Control of movement of trains

4.4 Signaling

(b) Absolute block system:or (Space Interval system) or (Lock and Block system)

'Absolute Block' refers to a system where the track is considered to consist of a series of sections, such that when

one train is occupying a section of track (the block section), no other train is allowed to enter that section.

In addition, no train can enter an empty block section without first securing the permission of the station in advance.

This is the most widely used system for ordinary train routes.

A station or signal box controls a block section in one direction (from its rear),

and no train may enter that block in that direction without permission from that signal box (the station or signalbox is said to accept or receive the

train). When a train has been accepted, no other trains can be accepted

on that block section until it has left that block section.

4.4.6. Control of movement of trains

4.4 Signaling

• Obviously the two signal boxes at either end of the block section have to tightly coordinate their actions,

• especially in the case of block sections that allow bidirectional movement on a single line.

• The permission to enter the block may be in the form of a physical token carried by the train crew

while the train is in the block, or may be implicit in the

aspects of signals governing access to the block.

4.4.6. Control of movement of trains

The section ahead was Absolute Block and through a tunnel

4.4 Signaling

(c) Automatic signaling or Automatic Block Signaling (ABS)

• the signals are automated and operate in conjunction with track circuiting or other means of detecting the presence of a train in a block section.

• When a train enters a block section, the stop signal protecting that block changes automatically to on or the Stop aspect.

• As the train moves ahead out of that block and into the next block, the signal aspect changes automatically to Caution.

• In multiple-aspect signaling, when the train is 2 blocks ahead the aspect then changes to Attention, and then to Proceed when the train has passed 3 blocks ahead.

• In 3-aspect signaling, the aspect changes to Proceed when the train is 2 block sections ahead.

• In automatic block territory 2-aspect signaling is not used.

4.4.6. Control of movement of trains

4.4 Signaling

• Each change of a signal to a less restrictive indication requires the train to have moved an adequate distance in advance of the signal.

• The gray, white, or silver boxes marked 'LOC CAB' found by the side of the tracks contain the circuitry to accomplish the automatic signal transitions.

• Any number of automatic block stop signals may be provided in between two block stations; thus with this system, the two stations do not define the ends of a single block section as is usually the case with manual absolute block working (excepting, of course, the case of intermediate block sections).

• Minimally one automatic stop signal is provided to the rear of a block station's first stop signal.

4.4.6. Control of movement of trains

4.4 Signaling

(d) Pilot guard system One individual- Pilot guard- accompanies the train ( may also give written

authority)- to the station ahead and returns after adequate time interval (15 minutes) to

the same station with another train. Combines the principles of one engine only and Following train.-used under specific occasions such as:-A failure of block-telegraph or telephone system in case of single line.-In double line, when one line is out of order and the other line is to be used for up and down trains.

4.4.6. Control of movement of trains

4.4 Signaling

(e) Centralized traffic control system:• Centralized traffic control (CTC) is a form of

railway signalling that originated in North America and centralizes train routing decisions that were previously carried out by local signal operators or the train crews themselves.

• The system consists of a centralized train dispatcher's office that controls railroad interlockings and traffic flows CTC territory.

• One hallmark of CTC is a graphical depiction of the railroad on which the dispatcher can keep track of trains' locations across the territory that the dispatcher controls.

• Larger railroads may have multiple dispatcher's offices and even multiple dispatchers for each operating division.

• These are usually located near the busiest yards or stations and their operational qualities can be compared to air traffic towers.

4.4.6. Control of movement of trains

controlled turnouts and signals utilizing computerized Centralized Traffic Control

Distant signalsA signal that provides advance warning of a stop signal ahead (and which

does not compel a stop when in its most restrictive position) is referred to as distant signal. The term originated in the UK and is used throughout the English-speaking world. In some regions, notably North America, the terms distant signal and approach signal are both in common usage.

Because of the long distance required to bring a moving train to a stand, distant signals must be located on the approach to the corresponding stop signal by at least the braking distance of the worst braked train to use the route. This is particularly important on high speed routes. At one time it was practice to take sighting distance into account when positioning distant signals; the distant signal could therefore be positioned at less than braking distance to the corresponding stop signal.

The driver of a train encountering a distant signal at 'caution' must expect the stop signal to be at 'danger' and must adjust the train's speed so as to bring the train to a stand before reaching it. The driver of a train encountering a distant signal in the 'clear' position knows that all applicable stop signals controlled by the same signal box are in the 'clear' position.

This is enforced by interlocking; the distant signal is prevented from assuming the 'clear' position unless all relevant stop signals controlled by the signal box display 'clear'.

A Finish distant signal at the western approach to Muhos station is displaying Expect Stop. In the background, express train 81 is pulling away from the station

4.4 Signalling

End position detector Decentralized point controller