Train control systems: signalling, interlocking and routing

Train control systems of various countries 

What is the main difficulty in stopping a train, compared to stopping a car? How is the fail-safe railway system achieved and what are the ways of railway control systems?

Railway systems employ various techniques and applications in order to safeguard safe and effective operation. First, let us make it clear why railway traffic is more difficult to control than roadways traffic.

The main difficulty in stopping a train, compared to stopping a car, is the adhesion available for a train with steel wheels on a steel rail. The contact area between the rail and the wheels is small and smooth. The weight of trains is also much larger. Which makes braking harder than cars.

The adhesion between a tire and the road surface can be measured at over 85%, while railways calculate their braking distances on the basis of 8% adhesion, which is an order of magnitude less. This makes it much more difficult for a train to stop in the right place and keep time, requiring skill and concentration.

Railway signals serve to provide route control indications to the train driver, allowing them to safely navigate the railway network. Signals indicate the status of the track ahead, whether it is clear or obstructed, and whether the train should proceed or stop. Signals are typically color-coded, with green indicating that the track ahead is clear, yellow indicating caution, and red indicating stop. Fixed speed limit signs are also present to indicate the maximum speed limit for movements over the crossover.

 Not every country, not even every railway within a single nation, uses the same procedures for signaling the protection of rail traffic. The type of  railway systems that is most nationally standardized is the signaling system. 

The quantity of traffic, the speed of the trains, and the traffic regulations in place determine when signals are needed and how many indications are required at each signal.

Rail systems include pointing devices and drailers, which are used to direct traffic in accordance with the movement authority, in addition to signals.

The technical capabilities for signaling have a significant impact on the methods used to regulate and direct railroad traffic. 

In most cases, the majority of signal systems are the outcome of successive evolution of technical development, where it has frequently been a challenge to incorporate new ideas without sacrificing the consistency of past achievements.

Types of railway signals

Signal appliances, which are further separated into fixed signals and signal signs, are used to produce the signal indications. Although all people involved in the movement of cars or trains are required to comprehend, obey, and use signals, they are principally directed at the drivers of engines and motor vehicles. 

A fixed signal’s function is to indicate whether a certain section of track beginning at the signal may be passed or not. 

Different signal light indications, the characteristics of which indicate the degree to which the movement is currently restricted by conditions within the track section itself or its continuation, grant permission to enter the track section. A fixed signal’s stop feature requires approaching trains to come to a complete stop before reaching the signal. 

The permission of the trains to enter a section of track depends on the track occupancy. When The condition of the tracks clearance can not be checked from the locomotive the driver must have the authorization to start before the movement begins. Such a motion signal is given by the person in charge of the operation of the trains by means of different types of signals and devices along the track. 

Railway signaling is a complex system of signals, signs, and different track devices that ensures the safe and efficient movement of trains on the tracks. Railway signals communicate vital information to train drivers about the track conditions ahead, including the presence of other trains, speed limits, and potential hazards. 

Different types of signals work together to ensure efficient railroad operation. Main signals, shunting signals, and signal signs are the common signal types in railway systems.

Main Signals

Main signals are the primary signals used to control train movements on the tracks. They are positioned at intervals along the tracks and communicate information to train drivers about the track conditions ahead. The most common types of main signals are the stop signal, the caution signal, and the clear signal.

Home Signal: The home signal is a signal sign that is positioned at the entrance to a railway station or a junction. It indicates to the train driver that they must stop and wait for permission to proceed.

Exit Signal: a signal that is located at the exit of a railway station or junction.

Distant Signal: The distant signal is a signal sign that is positioned some distance before the home signal. It indicates to the train driver the status of the home signal ahead.

Stop Signal: The stop signal is a red signal that indicates to the train driver that they must come to a complete stop before proceeding any further. The stop signal is typically used when the track ahead is occupied by another train when there is a hazard on the track, or when there is a speed restriction in place.

Caution Signal: The caution signal is a yellow signal that indicates to the train driver that they must reduce their speed and be prepared to stop at the next signal. The caution signal is typically used when the track ahead is partially occupied, or when there is a temporary speed restriction in place.

Clear Signal: The clear signal is a green signal that indicates to the train driver that they can proceed at normal speed and that the track ahead is clear. The clear signal is typically used when the track ahead is unoccupied, and there are no speed restrictions in place.

Shunting Signals

Shunting signals are used to control the movement of trains within railway yards or sidings. They are positioned at the entrance and exit points of the yard or siding and communicate information to train drivers about the status of the tracks ahead. The most common types of shunting signals are the shunt-ahead signal and the shunt-away signal.

Shunt-Ahead Signal: The shunt-ahead signal is a yellow signal that indicates to the train driver that they may proceed ahead into the yard or siding, but they must be prepared to stop at any time. The shunt-ahead signal is typically used when the track ahead is partially occupied or when there is a temporary speed restriction in place.

Shunt-Away Signal: The shunt-away signal is a green signal that indicates to the train driver that they may proceed away from the yard or siding at normal speed. The shunt-away signal is typically used when the track ahead is unoccupied, and there are no speed restrictions in place.

Signal Signs

The curvature of railways and other reasons many deem it necessary to limit the speed of trains. These are conveyed by particular signal signs. Other types of signal signs can be erected to convey information on the safety precautions for example when there is work going on or environmental conditions.

Speed Restriction Sign: The speed restriction sign is a signal sign that is positioned at the start of a speed restriction zone. It indicates to the train driver the speed limit for the zone ahead.

Railway Track Classification

Line and Stations 

A railway track falls into either the line or the station’s category. The main tracks of the railway can be single tracks or so-called single lines or can consist of double tracks or called double lines. A station is defined as any location on a railroad where trains utilizing the same main track are permitted to pass or meet.

 Stations consist of Train tracks that are utilized by coming or going trains. The lines of the main tracks start from about 200 yards from the furthest pair of points where the tracks converge into common tracks that connect the station to the lines.

The station includes the storage tracks that are joined to the train tracks. loading places can serve as places where storage tracks that are part of the line can also be joined directly to the main tracks.

Any place where the main tracks of different lines run together into a common main track is a junction situated on the line. In principle, movements within the limits of a station are 

safeguarded by inspecting the positions of the points and the unobstructedness of the tracks. 

A junction is a location on a line where the main tracks of many lines come together to form a single main track. The positions of the points and the clearness of the tracks are generally monitored to ensure that movements inside the confines of a station are protected. 

The safeguarding of the tracks between the stations and on the lines is secured typically in full or in part by station staff right before each train arrives or departs. This safety can be facilitated by track circuits and point interlocking, which automatically control the train’s movements.

Interlocking areas

While the basic idea of train separation by fixed block sections also applies on station tracks, many railways separate the station areas from the open line (see the following paragraph on interlocking areas) using the term block section only outside of station areas. Main tracks or running lines are used by main train movements. The lines between stations and their continuation through stations and interlocking areas belong to this category. Loops which are also considered main tracks as are used for overtaking. However, sidings are used only for shunting moves. 

Interlocking Areas in Railway Tracks

If you have ever traveled on a train, you might have noticed the signals along the railway tracks. These signals help the train operators know when it is safe to proceed and when they need to stop. But did you know that there is a lot more to these signals than meets the eye? we will discuss the concept of interlocking areas in railway tracks and how they work.

What are Interlocking Areas?

An interlocking area is a section of a railway track that has controlled signals. These signals are interlocked with points and other signals in a way that ensures that the train can only move when all points are locked in the proper position, and all conflicting moves are locked out. The signals that control interlocked routes are called interlocking signals. These areas are controlled either by a local interlocking station or from a remote control center.

Types of Interlocking Areas

There are two types of signal arrangements in interlocking areas. First, there are interlocking areas without consecutive interlocking signals. This means that an interlocking signal provides authority to run through the entire interlocking area into the next block section. 

Second, there are interlocking areas with consecutive interlocking signals. Such an interlocking area may contain tracks protected by controlled signals on which trains may originate, terminate, pass, and turn. These tracks are called station tracks. An arrangement of station tracks is called a station area.

Home Signals and Exit Signals

On most railways, the interlocking signals that protect a station area from both sides are called home signals. The signals that govern train movements to leave a station track into a section of the open line are often called station exit signals or just exit signals.

 Other commonly used terms for these signals are section signals, starter signals, or leaving signals. Interlocking signals within the station area that are neither home nor exit signals are called intermediate interlocking signals.

Difference between platforms and stations

It is to be noted that A platform station used for scheduled stops of passenger trains may not always require a dedicated station track and a station track may not have a platform. 

Moreover, there are platform stations located on the open line that may not be provided with pointwork or the capability to reverse trains within the signaling system.

North American and British naming of station tracks and tracks

Interlocking areas with consecutive interlocking signals within the same interlocking limits are not prevalent in North America due to the absence of a distinction between station tracks and tracks of the open line in North American rules (Bisset et. al., 2008). 

However, station areas with consecutive interlocking signals are commonplace in Europe and most other railways outside of North America. In modern British signalling centers, there is no longer a formal differentiation between station tracks and sections of the open line. 

How to safeguard the traffic on main tracks between stations

The safeguarding of the traffic on main tracks between stations is based on the so-called movement authority. Movement Authority (MA) is an essential permission that indicates a train’s freedom to move from one point to another while taking into account the peculiarities of the railway infrastructure.

There are different types of movement authorities from the simplest using train personnel to complicated centralized control centers. 

The manual and most basic movement authority is the control of train movements is managed by train personnel located in the train stations.

 The purpose of the movement authority using train personnel also called signalmen is to prevent more than one train at the same time from occupying the main track or each one of the block sections into which the main track may be divided. 

The departure of a train from a station or a block post is announced by telephone to the next station or block post, from which the arrival of the train is reported back to the first mentioned station or block post. Until this report has come in the next train must be held back. At single lines, permission must also be obtained from the next station or block post just before the train is dispatched. 

Train personnel or signal men may not be required on lines where the use of the main tracks is governed by automatic block signals, even if signal men may control the signalling in some cases.

At junctions, loading places, and other traffic places on the line the movements may be secured by personal inspection of the tracks as at stations. However, the manning of such places can be avoided by locking the points either manually from the stations or automatically by means of track circuits. This mechanism is also called interlocking points. In either case, particularly on principal lines, the proper positions of points on the track must be indicated to the drivers by fixed signals. 

Types of Movement Authority

There are different ways that trains can receive MA, as following

Movement Authority from Signalmen

In most forms of train control, the movement authority is passed from those responsible for each section of the rail network, such as a signalman or stationmaster, to the train crew. This process uses a set of rules and physical equipment that determines the method of working, operation, or safe working.

Movement Authority from Central Apparatus

The Central Apparatus is a device used to control train movements from a central location. It can provide MA to a train according to the characteristics of the infrastructure and the freedom of the street, such as line peculiarities, signaling, slopes, speed limitations, the position of the downstream trains, etc.

Movement Authority from a Blocking System

A blocking system is a device that controls the movement of trains through a section of track. It can provide MA to a train based on the availability of the section of track and the location of other trains in the area.

Movement Authority from a Radio Block Center (RBC)

An RBC is a device used to control train movements through a wireless system. It can provide MA to a train by transmitting the authorization through radio waves, either automatically or upon request by the train.

Movement Authority from Discontinuous (Balise) or Continuous (Loop) Information Equipment

Balise and Loop Information Equipment are devices that provide information to trains through physical transmitters on the track. They can provide MA to a train based on the information received, such as location, speed, and signaling status.

Train and Shunting movements 

A regular train movement takes place when an engine or motor vehicle, whether alone or with other vehicles coupled with it, is moved from one station to the next one, which takes place after a timetable specifying the composition of the train and the necessary brake equipment has previously been approved by the train despatcher. 

The train movement begins when the train starts from its departing station and ends when the train comes to a stand in its final station. If the train stops at an intermediate station the train movement becomes for the time interrupted but begins again when the train departs. Train movements chiefly take place on the line. Within stations train movements to occur only in connection with the trains’ arrival from or departure into adjoining lines.

Train movements mainly occur on the lines or running lines(UK) or main tracks (USA). e. The lines between stations and their continuation through stations and interlocking areas belong to this category. It also includes tracks for passing and overtaking trains which are called loops on most railways

All other movements with railway vehicles, whether within the limits of a station or on the lines, are regarded as shunting.

 Shunt movements also are regarded as movements of trains for short stretches after coming to a stand in stations or on the line, as well as any movement in the direction opposite to the one in which the train normally is traveling, regardless of what causes the backward movement. 

In sidings only shunting movements occur and interlocking is not provided. The maximum speed at shunting is limited to about 20 miles an hour. 

Note: In North American terms, a line is a route that may consist of several parallel tracks. Furthermore, loops are called sidings; tracks other than main tracks are called yard, secondary, or industrial tracks. Single-end tracks connected to a main track are called spur tracks.

Authorization of train movement

The authorization of a train movement has two elements:  

-A valid timetable as the authority to run through the network along a pre-defined route by specified operating conditions (timetable authority)  

-A movement authority for every single section of track in the path of the train 

The movement authority to enter a section of track is issued by the operator who is in charge of controlling train movements on that section of track. This way, a train is always under the external guidance of a train control operator. 

The authority for train movements is given by:  

-A proceed indication of a main signal 

– A proceed indication of a cab signal display  

-A call-on signal permitting a train to pass a signal displaying a stop aspect under special conditions  

-A written or verbal instruction permitting a train to pass a signal displaying a stop aspect under special conditions  

-A written or verbal authority on non-signal-controlled lines 

Authorization of shunting movement

The authority of shunting movements is given by: 

– A proceed indication of a shunting signal, which may be combined with a main signal to authorize a shunting move to pass the main signal in the stop position  

-Verbal permission

Train Routes

First, let us explain what paths and routes are on railway tracks. The path is a term used to denote the actual possible way on a railway in a certain condition. The railway points set the actual path of a railway. Some possible paths [1]. Paths are arranged and all movable elements on it are locked to safe train movements by an interlocking system. This safe path is called a “route” . Every route has a starting and exit signal

The stretch of track over which a train has to run upon entering a train track at a station is called the entrance route of that train. The route begins at the station limit and ends on the 

train track at the place to which the train can advance without interfering with any of the entrance routes from the other direction. When the train is allowed to enter without the track 

being entirely free up to the normal end of the entrance route, the train is said to be using a curtailed entrance route. 

The stretch of track over which a train travels on leaving a train track for a line is called the exit route of the train. This commences where the entrance route ends and ceases at the 

station limit. 

A train route may belong to the line and include for instance the main track through a junction, a pair of points at a loading place, a movable bridge, or any other obstacle that can impede the traffic and therefore must be scanned before trains are allowed to pass. At junctions, each route is considered as belonging to the line between those stations which get connection over the route in question. 

Block Sections 

Block sections are used as units in the control of train movements on the route between two adjacent stations. The rail routes in the lines mentioned above are included in the block sections of the tracks. A block or several block sections may make up the routes. Multiple block sections are used to boost the traffic capacity so that more than one train can be sent as far as possible in the same direction. Fixed signals are available at the beginning and end, as well as in between the blocks, to regulate train movement in the block sections.

Trains can also move in both directions if the movement is permitted by the train controllers from the station on both sides of the block.

If the movement is approved by the train controllers from the station on both sides of the block, trains may likewise move in both directions.

A train must not be followed by another train in the same direction before the last vehicle of the first train has passed B if the main track between the two station limits A and B contains just one block section for each direction of traffic. 

The time interval between the trains will be equal to the amount of time that the first train occupies the track between points A and B, plus any additional time that may be needed for the second train to reach station limit A. 

The distance required on advancing to A must be considered.

1. For trains scheduled to call at the station, the distance to be traveled on the way to point A must be calculated from the location where the train is standing at the time of departure.

2. For trains that do not stop at the station, the distance must be calculated from the location where a signal allowing the train to pass the station must be sighted in order for the driver to be able to approach A at a normal speed. 

The dividing of the track into multiple blocks shortens the time intervals between trains traveling in the same direction. The main track between A and B could be divided into two or more block sections, such as Ax, xy, and yB, where x and y are line-based intermediate block posts. 

The time interval between the trains entering the line at A will be equal to the time it takes required for the first train to move from A to x increased with the time required for the second train to advance to A.

 Similar to this, the required time interval between two trains at x, or y, must be equal to the amount of time needed for the first train to cross-section xy, or yB, plus the time needed for the second train to move toward x, or y, as appropriate. The density of traffic between stations A and B is determined by the longest time interval acquired in this manner. 

The time intervals mentioned above are only applicable to trains traveling in the same direction. Whether there are block posts or not, the time interval between trains traveling in opposite directions is equal to the time it takes the first train to travel from point A to point B, plus the additional time required for the first train to enter the station at point B and the second train to move forward to the station limit at point B. 

Block control methods

Railway signaling is a crucial component of train operations that ensures the safety of passengers, crew, and infrastructure. It refers to the various systems, equipment, and protocols used to manage train movements and maintain safe distances between trains. 

The purpose of railway signaling is to prevent collisions, derailments, and other accidents that may occur due to human error, equipment failure, or other factors.

To ensure safe train separation, the control procedures must ensure that the movement authority to enter a section of the line must not be issued unless two basic conditions are in effect: 

1. The line is clear up to the desired authority limit and the rear end of the last train ahead is safely protected by that limit. 

2. All opposing moves on the same stretch of the line are safely locked out. 

While these basic safety requirements are valid both in fixed block and moving block(cab signaling) systems, the solutions to meet these requirements differ.

Block control by verbal communication

On branch lines operated at a low speed and a very low traffic density, train movements may be protected just by operating rules under staff responsibility. This is based on verbal communication by radio or telephone

These systems are classified as dispatcher-controlled and operator-controlled systems.

Dispatcher-controlled operation:

In dispatcher-controlled operation, the dispatcher is responsible for train control on a long stretch of line. The dispatcher communicates with train crews via radio and the crews manually operate the points. The train crews report their arrival at stations or specific locations. 

The dispatcher keeps track of all movements by manually recording messages on a paper train sheet or entering them into a computer system. Movement authorities issued by the dispatcher are transmitted verbally by radio and recorded manually. 

Train locations and movement authorities are visualized on a computer screen. That control system is just an offline system controlled by the data manually entered by the dispatcher, 

However. On lines with a higher density of traffic, a simplified signaling system may be used as a safety overlay. In such a system, automatic block signals controlled by track circuits would protect occupied sections of the line. Since there are no controlled signals, movements authority has still to be issued verbally by the dispatch

Train control by local operators:

Train control by local operators is still common on some railways, although it is less frequently used today. On such lines, all stations that limit a block section must be locally staffed.

 The local operators communicate by telephone and exchange control messages on trains entering and leaving the block section, as well as direction control to protect opposing moves. The exchanged messages and train movements are manually recorded. 

Movement authorities are usually issued by lineside signals controlled by the local operators. Under very simplified conditions, stations are only equipped with home signals, and the authority to leave the station into the next section of the line is given verbally or by a hand or flag signal.

There are several types of railway signaling systems, each with its own set of principles and methods. Some of the most common types of signaling systems include manual block systems, automatic block systems, and centralized automatic block systems. Let’s take a closer look at each of these systems.

 Block Systems for Fixed Block Operation

 To protect a train that has entered a block section against the following trains, the signal at the entrance of the section is locked in a stop position. The signal can be either a lineside signal or just a section limit (usually marked by a block marker board) where a train must not proceed without a cab signal indication.

 If converging lines lead into the same section, the block-locking is in effect for all signals leading into that block section. After the train has completely left the block section including the overlap (if overlaps are required) and is protected by a stop signal, the block section is released. Now, a signal at the entrance of the block section can be cleared for a following train 

If the line just consists of one single block section, opposing moves can be easily locked out by locking all opposing signals in a stop position before a signal to enter the section can be cleared. 

Manual Block Systems

A manual block system is a basic signaling system that uses physical barriers and signals to manage train movements. In this system, the block sections are not yet equipped with continuous track clear detection. The signals protecting the block sections are manually controlled by local operators. The stations are equipped with electric block instruments connected by a block line.

 After a train has entered a block section, the operator at the entrance of the section would restore the signal and operate a block instrument to lock the signal in the stop position. A coacting instrument at the exit side of the section informs the next operator on the approaching train.

If the operator at the block entrance fails to operate the block instrument, a rotation lock would lock the signal in the stop position until it is properly locked by the block instrument. This system is used on branch lines with low-speed operations and low traffic density, where train movements can be protected by operating rules under staff responsibility.

Automatic Block Systems

An automatic block system is a more advanced signaling system that uses continuous track clear detection to enable the signals to work automatically. Block sections are equipped with automatic track clear detection to enable the signals to work automatically. An automatic signal will only clear if the entire control length up to the clearing point beyond the next signal is clear and a train ahead is protected by a stop signal. To release a block section, a train must not only have cleared the section and – if required – the overlap but must also have restored the next signal. This condition confirms that the train has safely passed the exit side of the block section.

Concerning the control principle, automatic block systems can be divided into two classes: Decentralised automatic block systems and centralized automatic block systems.

In a decentralized automatic block system, the control devices are located in field cabinets directly at the block signals. These block signal cabinets exchange block control information either by an electric block line or through coded track circuits. After a train has left a block section, the signal at the entrance of that section is immediately cleared. So, in a normal state, automatic block signals that are not held down by direction locking are cleared no matter whether a train is approaching or not. On bidirectional lines where opposing moves are protected by direction locking, all signals pointing against the locked direction are locked in the stop position.

In a centralized automatic block system, the block control is part of the centralized control system that also controls the interlocking areas. Instead of exchanging block control information, centralized block sections are treated similarly to routes in an interlocking system. However, these ‘block routes’ do not lock any points or movably parts of the infrastructure. They are just part of the block control logic.

Railway track train detection systems

Train detection devices are used to establish whether a specific stretch of track is being used by a train. Train detection systems come in two primary categories: manual detection and automatic detection. While automatic detection uses track circuits or axle counters, manual detection includes the signaller looking out the window of the signal box at the tracks. 

Automatic detection is used in automated blocking systems, where the signals are automatically operated by relays attached to track circuits. Hence, track circuits and axle counters make automatic block control and signaling possible.

Track circuits use insulated rail segments to create an electrical circuit, which flows through the wheels as the train enters the track section. The closed circuit theory underlies this system, according to which any presence of the current will result in a safe condition by making the section occupied. 

On the other hand, axle counters work by simply counting the axles of a train as it enters and exits a segment of the track. Any net number of axles in the section suggests that the track segment is occupied if it is initially clear.

An important idea in railway signaling is the fail-safe principle. The purpose of fail-safe systems is to guarantee that, in the event that a signaling system component fails, the system will immediately switch to a safe state.

 For instance, even if there isn’t a train present, the system will detect a track circuit failure and believe that the section is occupied. Every component of the signaling system includes fail-safe devices, which are essential to assuring the safety of railway systems

Due to their immunity to electromagnetic interference (EMI) from electric trains and the lack of requirement for electrically insulated individual rail segments, axle counters are becoming the preferred method of train detection rather than track circuits. For track circuit detection in modern installations, track relays are frequently replaced by electronic detectors.

Before a train is authorized to run along a route, track clear detection equipment is used to verify that all pertinent track sections are free of train vehicles. Axle counters and track circuits can both be used to identify if the track is clear.

  Specific operating rules must be followed when using non-shunting vehicles and when rusted or unclean rails hinder safe working. For instance, staff members are responsible for protecting non-shunting cars that cannot reliably produce an electrical connection between the rails by securing protective signals in the stop position.

 Similarly , even regular vehicles will no longer be safely identified if the rails of a track circuit portion are severely corroded. In order to maintain the safety of the track circuits, several railways have set regulations that call for a minimum number of movements within a given amount of time.

Type of Interlocking systems

As you ride a train, you might have noticed that the signal lights and switches along the tracks are always on the move, indicating the direction and speed of the train. These signals are not controlled randomly but by complex interlocking systems that ensure the safety of railway transportation. In this blog post, we will explore the different types of interlocking systems used in railways, from the oldest mechanical systems to the modern computer-based ones.

Mechanical and Electro-mechanical Interlocking Systems

The oldest interlocking systems in railways are the mechanical and electro-mechanical interlocking systems. These systems are controlled by lever-frame machines, where the levers of points and signals are mechanically interlocked.

 The development of mechanical interlocking systems dates back to the late 19th century, and they are still in use in some countries today. In mechanical interlocking, points and signals are operated by the muscle power of the local operator, and the levers are connected to the controlled track elements by mechanical wire or rod transmission. To compensate for the lack of electrical monitoring of the point positions, facing points are often equipped with an independent point lock, which is operated by a separate lever.

The electro-mechanical interlocking systems were developed at the beginning of the 20th century. In these systems, points, and signals are controlled either by electric motor drives or electro-pneumatic drives. The lever frame consists of miniature levers that are actually electrical switches but are also still mechanically interlocked. Point positions are electrically monitored.

Relay Interlocking Systems

In relay interlocking systems, the control logic is realized by relay circuitry without any mechanical elements. The circuitry may be based either on tabular interlocking with a free-wired logic or on geographical interlocking. 

Points and signals are no longer operated by levers but by simple push buttons usually located in an illuminated geographical track diagram. The development of relay interlocking systems started in the 1920s and was the dominating interlocking technology in the second half of the 20th century. They are still in use in many countries today.

Computer-based Interlocking Systems

The latest and most modern interlocking systems in railways are computer-based interlocking systems. In these systems, the control logic is represented by software, and the track elements are controlled by computer-based interlockings.

 The first computer-based interlocking systems were developed in the 1980s, and they became the preferred technology for new installations in the 1990s. Nowadays, computer-based interlocking systems are almost exclusively used in all new installations, but older generations of interlocking systems are still in use, especially in some countries.

Train Control by Signal Sections 

The driver of a manually operated train has challenges due to the lengthy braking distances required by trains. He is unlikely to be able to spot an impediment or detour that would force him to slow down or stop the train. He is essentially traveling “blind” He (or she) needs advice before the point at which the brake needs to be applied in order to overcome this. Railways have developed a system that manages this problem, which we call signaling

It is typically not essential to open up entire routes or block sections for the movements when shunting is done in train routes or block sections. Shorter track units are usually sufficient. 

Signal sections are used to split train routes and block sections to satisfy the unique needs of shunt movements. However, the shunting does not always travel through the track layouts in the same manner as the trains. For this reason, signal sections that diverge from train routes or are located on lines other than the train routes, or block sections are frequently needed. 

A railroad layout’s division into signal sections must be carefully studied case by case and depends heavily on local circumstances. Short signal sections facilitate traffic flow by allowing different movements simultaneously. On the other hand, any increase in the number of signal sections tends to drive up the price of installing signals. Therefore, it is important to carefully assess each signal section’s necessity. 

Pulling vehicles from a train track onto an exit route is frequently the first step in the shunting of the train routes at stations. The amount to which other movements would be freed up by opening only a portion of the route for such shunt movement that does not require the occupation of the entire route determines whether the exit route should be separated into signal portions. 

The inconveniences can be greatly reduced in stations where the train tracks are too short to accommodate trains of all common lengths by segmenting the exit routes into signal sections. This will allow longer trains to move beyond the ends of the entrance routes as needed to speed up trains or shunt movements behind the trains. 

Shunting in the entrance routes of a station is required. 

(1)When a locomotive or a group of vehicles arriving from the line are to be brought into the station without the movement being treated as a train, or(2) when vehicles have been pulled out of the station and are to be pushed into a train track, 

In the first scenario, it may be necessary to have a signal section that starts at the station limit and ends at the furthest pair of points in order to manage the shunt movement. At these locations, the movement can be stopped while waiting for the next signal section that enters the train track to be clear. 

the last mentioned signal section, starting at the outermost points, may also be helpful in the second scenario. The benefits of allowing the pushing back of the vehicles to start before the locomotive, drawing the cars out of the station, has passed beyond the outermost points will determine if the entrance path needs to be divided somewhere between these points and the train track. 

By leveraging the technical tools at our disposal, interlocking points can be released for operation behind moving vehicles as soon as the final axle has passed the points, thereby reducing the number of signal sections necessary. So, before all the cars have left the signal section utilized for the prior movement, the route can be readied and the proceed signal displayed for a subsequent backward movement. 

It is possible for many signal sections to start at the same track and end on different tracks. Similar to this, several signal sections that start on different tracks may converge onto a track that is shared by all the sections. Only one motion direction may be covered by each signal section. Other signal sections are employed to control movement in the opposite direction. Even though the positions of all points remain unchanged, when motion is reversed after entering a signal section, the cars are actually moved into a new signal section. 

Guiding Trains in Fixed Block Operation 

While, with the introduction of a radio-based train control system, railways move more and more toward cab signalling, lineside signals are still the dominating form of train control. 

They are even used in many new installations. Since lineside signals can only transmit movement authorities at fixed intervals, train control by lineside signals always leads to a fixed block operation. For this, the line is divided into block sections limited by signals. 

To clear a signal for a train that is to enter a block section, the following conditions must have been fulfilled 

 The train ahead must have cleared the block section 

 The train ahead must have cleared the overlap beyond the next signal (only on lines 

where block overlaps are used) 

 The train ahead must be protected by a stop signal 

In line with the bidirectional operation, the train must also be protected against opposing 


Train control by interlocking

Interlocking is a crucial safety measure implemented when a train passes through a point zone. This ensures the train movement is secured, as apart from the distance between trains, it involves securing movable track components in their correct positions and shielding the train against any conflicting movements that may impede its path.

 This is achieved by integrating the signals controlling train movements with those governing movable track components and conflicting routes, hence the term “interlocking.”

Interlocking Routes 

The principle underlying interlocking is that a safe path up to the authorized limit must be established before a train can be allowed to pass through a point zone by either clearing a lineside signal or issuing cab signal authority.

That route must meet the following conditions: 

 -All points must be properly set and locked. 

 -All points must be kept locked as long a train has the authority to move on them. 

– Conflicting moves must be locked out. 

– The route must be protected against inadvertent movements on converging tracks 

(flank protection). 

 -All track sections the train has authority to pass through, must be clear. 

Route Classes 

Modern interlocking systems are provided for shunting movements as well as main train movements. However, there is a distinction between the main and shunt routes..

Main routes are regular train movements governed by a main signal or cab signal indication. 

Shunt routes are used for shunting movements authorized by a shunt aspect or verbal authority. 

A shunt route is exempt from some of the regulations that apply to major routes. Therefore, a shunt route may control a shunting movement into an occupied track. Typically, flank protection for the shunt routes is either omitted entirely or simplified. 

The same interlocking routes may be utilized for both train and shunting movements in North America because there is a weaker separation between the two types of movements.

A main route starts always at a controlled signal (the entrance signal of the route). The exit of a route can be: 

 -The next controlled signal (the exit or destination signal of the route) 

 -The end of the interlocking area 

Routes between subsequent signals within the same interlocking region are those that have both an entry signal and an exit signal. And, the points within the overlap beyond the exit signal are interlocked with the entry signal on railways where overlaps are necessary

This type of interlocked route also directly provides a safe train separation since the interlocking system checks the clearance of the segment between the entrance and exit signal. 

Train movements to depart the interlocking area are governed by routes with an exit at the end of that area. A safe train separation cannot be guaranteed on such a route. The path enters a part of the line that is block-protected.

 In North American interlocking systems, the route will always end at a controlled signal facing in the opposing direction which limits the interlocking as shown in the figure above. In North American terms, this signal is called an ‘exit signal’. That term should not be confused with the use of the term ‘exit signal’ for routes between successive 

controlled signals of the same direction as explained above.

 In North American interlocking systems, the route always comes to an end at a controlled signal that faces the interlocking’s limit as in the figure above. This signal is known as an “exit signal” in North America. The term “exit signal” for routes between successive controlled signals in the same direction, as previously described, should not be confused with that phrase.

 A track section beyond the route’s final points serves as the exit for such a route on continental European railroads. This route exit is not always associated with an opposing controlled signal, contrary to North American customs.

 A signal is always present at a route’s end on British railroads. There a route that leaves an interlocking area to an automatic block line is provided with the first automatic block signal.

The figure below shows a more detailed view of the elements of a route between two successive interlocking signals. The details will be explained in the following sections. 

Point locking 

 All points and derailers must be secured into place before a signal can be cleared. 

There are two components to locking points: 

 To stop the points from operating 

 To avoid the point blades under a moving train from moving improperly 

The interlocking system’s internal logic ensures that the first requirement is met. All points have a point lock apparatus that mechanically locks the point blades in the right position in order to satisfy the second requirement.

Locking and releasing Routes 

All route segments must be kept locked after the train has passed the signal until it has cleared them or has come to a safe halt. 

The idea behind approach locking serves as the foundation for route locking on many railways. The route is simply locked by the cleared signal as long as no train is approaching. 

When restoring the signal, the route will immediately release. When an approaching train has reached a position at which canceling the route would change a signal aspect in front of the train, the route will be approach locked. That is to prevent the route from being changed within the safe braking distance on the approach to the signal.

The route will instantly release when the signal is restored. The route will be approach locked once an incoming train has passed the point at which canceling the route will modify a signal aspect in front of the train.

 After the signal has been restored, approach locking will now keep the route locked. 

Once the train has passed the signal, the normal route locking kicks in and keeps all elements locked under the moving train. 

Some railways, in particular, railways that follow German principles, do not use approach locking but establish full route locking immediately when clearing a signal independently from an approaching train.

Some railroads, particularly those that adhere to German principles, implement full route locking right away when a signal is cleared independently of an approaching train instead of using approach locking.

The normal release of route locking is usually attained automatically after the train has cleared the points.

 A sectional route release or a complete route release with a single clearing point can both be used to implement the automatic route release. The train needed to be on the destination track and have completely cleared the point zone in order for the route to be released completely.

 The use of sectional route release requires Track components that could release separately that must have separate sections for track clear detection.

 They must have a minimum length that is greater than the greatest distance that can be between two axles in the train in order to prohibit independently releasing track portions to be cleared before the train has passed through. 

Separately releasing track sections are joined via a process known as sequence locking for safety reasons. This stops the release of a track element until the release of the preceding element. 

Where overlaps are offered, the points that fall within the overlap have time release. As the route exit signal approaches and the train occupies the final length of the track, they will automatically release with a predetermined delay. 

It may happen that a locked route has to be manually released under staff

responsibility if the route has either to be canceled without having a train passed or if the normal route release failed after the passage of the train. For the latter case, most railways provide an emergency route release. 

Route cancellations and emergency route releases are protected either by time locking or by a specific command procedure with automatic recording.

A route may need to be canceled without a train passing through it, or it may fail to release normally after the train has passed, in which case staff may need to manually open the route. Most railroads offer an emergency route release for the latter scenario. 

Route cancellations and emergency route releases are secured using either time locking or a particular command process with automatic recording. 

Flank Protection 

Vehicles on convergent tracks or in flank zones should be shielded from the movement of trains by flank protection. This might be done by:

-Operating regulations

-Flank protection equipment 

According to operating regulations, particular station tracks cannot be used for equipment storage or shunting as long as the route is established in order to safeguard a primary route from unintentional movements on tracks that connect the train’s path.

 Because it is not particularly effective, this type of flank protection should only be employed when none of the other options are available. 

Trackside elements that are controlled are used as flank protection devices. The flank protection devices that are employed normally are flank points, derailers, or stop signals. 

Flank protection devices are controlled trackside elements.

Stop signs are only adequate for flank protection from driver-controlled movements. Flank protection must be implemented by flank points or derailers to safeguard a train from objects that could accidentally go into motion (for example, on lines where equipment is stored) or from the flank hazard from tracks with significant shunting. 

Derailers cannot be erected outside of sidings on many railways. Flank points usually provide flank protection for high-speed lines against shunting maneuvers and parked machinery. 

Remote Flank Protection 

If at all possible, elements that are immediately adjacent to the route that needs to be protected should be used to offer flank protection. Elements farther from the point to be protected may be used to provide flank protection if a suitable element is not available. Remote flank protection is the term used to describe this type of flank defense as shown below.

Since they do not really defend but rather transfer flank protection, the points between the protective elements and the points to be protected are known as flank transfer points. The protective pathways may diverge at flank transfer locations, protecting one route element from multiple directions. 

Front protection

This kind of protection is used to guard the route against unauthorized cars traveling in the opposite direction. A front protection element is represented by the point in Figure 3.16.In most cases, it is necessary when the overlap distance is not so long

Signal blocking for routes

Signals for flank movements can be stopped if there is no point or derailer device. When points are unable to provide flank protection, they pass the function to signals or other points. Providing remote flank protection is what this is.

Conflicting routes

Conflicting routes are those that utilize the same track elements. One route can only be set at a given moment, and the others must be blocked. The figure below displays a few intersecting pathways on various tracks.

Route setting

Main route setting

One of an interlocking system’s fundamental functions is route setting. Route-setting processes can vary between interlocking systems. The following list of fundamental and significant main route-establishing steps is explained: 

1. Checking for conflicting routes: When a route-setting is requested by a signaling operator or an automatic route-setting device, the interlocking system looks for any routes that are already set in conflict with the desired route.  

2.  Failure checking: The interlocking system verifies all trackside equipment connected to the necessary route as a second phase. The route setting request cannot be authorized if a trackside equipment breakdown is discovered.  

3. Checking for occupancy: The route path and any overlap must be free of any occupied rail sections. Some track sections in the flank area of some routes must also be free.

 4. Setting moveable components: The final step is to set them in their proper positions and lock all movable elements (in the route path, flank area, and overlap). Any element that has been previously locked in the incorrect location or that is not under remote control cannot be set, and the request to set the route is refused. Additionally, when the elements are moving, it is also possible to receive a failure message from them. As a result, requests are canceled. Additionally, any occupancies on the points in the flank protection region are grounds for route-setting requests to be denied.  

5. Locking the route and signal opening: If all of the previous procedures were successful, the route could be locked and the signal could be opened correctly based on the circumstances of the subsequent signal. All route specifications need to be set in the interlocking system in order to put these regulations into practice. For example, one approach for route definition is the development of a table. We’ll discuss the route table later.

Shunting route setting

Shunting routes, as opposed to main routes, can be set when there is some path occupancy. However, if a track segment is occupied and the path cannot be determined, incorrectly placed points cannot be set.

 Additionally, in some interlocking systems, flank protection and overlap are not taken into account for shunting. The remaining guidelines for main routes and shunting routes are nearly identical.

Route releasing and reversing

After the trains traverse the running path, the route is released. Track sections in the path should follow a set-occupied-free sequence. Set-occupied-free sequence, illustrated in Figure below., is defined to represent a normal train movement in a main route.

Different route-releasing conditions can be described for particular routes. Following 

steps are described as a general main route terminating procedure. When the first track section is occupied, the start signal has to show the “stop” aspect immediately.

The passage is opened up when the trains have passed through the running path. Track segments in the way ought to be organized in a set-occupied-free pattern. A typical train movement on a main route is characterized as the set-occupied-free sequence shown in the Figure below.

For certain routes, several route release conditions might be provided. An overview of the general main route termination technique is provided below. When the first track segment is occupied, the start signal must instantly display the “stop” aspect.

 Until the train departs the final track section, all equipment at the side of the tracks must be kept locked. After that, they can be released. Some equipment, nevertheless, can be unlocked after the train has passed over it for operational reasons.

All flank protection objects that are kept locked may result in operational delays. 

For flank protection objects, special unlocking procedures are established.

The process for terminating a shunting route is quite similar to that for terminating a main route.

If there are no trains approaching the starting signal, the route-setting can be simply reversed for operational reasons. However, for safety concerns, a train cannot be promptly canceled as it approaches a route. The interlocking system waits before terminating the route. The route is released if the train does not arrive in the routing area within a certain time.

Route table

The interlocking table, also known as the route table, is made up of a list of all the specifications for every potential path in an interlocking system. It displays each piece of equipment’s necessary status along a particular path.

 A route table is developed by the system designer as one of the initial steps when creating a new signalling system, and all systems are designed based on it. 

Route tables do not have a common form. Each system designer uses a different form for their route table. For the straightforward layout displayed below, a sample route table has been developed.

A route table that was created displays the positions of trackside elements for each route. 

Route names are listed in the first column. The starting and exit signals for the routes are listed in the second column. For instance, Route 1 begins at signal 7 and ends at signal 1, 

The track sections on the path and on the flank areas are displayed in the “Track sections” column. Track segments in flank areas must also be free of obstructions for the safety of the routes, as was previously mentioned. The “Points” column is designed to provide the names and proper positions of the route’s points. 

The “Flank protection” column displays objects used for flank protection. 

The required positions of the points are indicated by the notations “(N)” and “(R)” (N: normal, R: reverse). Overlaps and spots used for front protection in the associated routes are noted in the “Front Protection” column. In the final column, conflicting routes are listed.