Railway signalling
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Railway signalling is a system used on railways to control traffic safely, for example, to prevent trains from colliding. Trains are uniquely susceptible to collision because, running on fixed rails, they are not capable of avoiding a collision by steering away, as can a road vehicle; furthermore, trains cannot decelerate rapidly, and are frequently operating at speeds where by the time the driver/engineer can see an obstacle, the train cannot stop in time to avoid colliding with it. This necessity was at the base of the establishment of strict guidelines for time keeping and railway chronometers in 1891 by the general time inspector Webb C. Ball of Cleveland, Ohio, USA and in 1889 by the UK parliament passing the Regulation of Railways Act 1889 - a series of requirements on matters such as the implementation of interlocked block signalling and other safety measures as a direct result of the Armagh rail disaster in that year.
Most forms of train control involve movement authorities being passed from those in charge of the rail network or portions of it (e.g., a signalman or stationmaster) to the train crew. 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 safeworking (Aus.). Not all methods require the use of physical signals and some systems are specific to single line railways.
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[edit] Timetable operation
The simplest form of operation in terms of equipment at least, is operation according to a timetable. Everything is laid down in advance and every train crew knows the timetable. Trains can only operate in pre-arranged time periods, during which they have 'possession' of the track and no other train can operate.
When trains are operating in opposing directions on a single-line railroad, meets are scheduled, where each train must wait for the other at a point they can pass. Neither is permitted to move until the other has arrived.
The timetable system has several disadvantages. The first is that there is no positive confirmation that the track ahead is clear; only that it should be clear. This system does not allow for breakdowns and other such problems. The timetable is set up in such a way that there should be sufficient time between trains for the crew of a broken-down or delayed train to walk back up the line far enough to set up warning flags, flares and the explosive devices known as detonators or torpedoes (UK and US practice, respectively) which alert a train crew to a blocked track ahead.
The second problem is the timetable system's inflexibility; trains cannot be added or delayed; trains cannot be rescheduled.
The third is a corollary of the second; the timetable system is inefficient. To give a little flexibility, the timetable must give trains a broad swath of time to allow for some delay. Thus, the line is possessed by the train for much longer than is really necessary.
Nonetheless, this system permits operation on a vast scale, with no requirements for any kind of communication that travels faster than a train. Timetable operation was the normal mode of operation on American railroads in the early days.
[edit] Timetable and train order
With the advent of the telegraph, a more sophisticated system became possible because the telegraph provided the first system available where messages could be transmitted faster than the trains themselves. The telegraph allows the dissemination of alterations to the timetable, known as train orders. These override the timetable, allowing the cancellation, rescheduling and addition of trains, and almost anything else. Sufficient time must be given, however, so that all train crews can receive the changed orders.
Train crews generally receive the orders at the next station at which they stop, or sometimes orders are handed up to a locomotive 'on the run' via a long staff. Train orders allowed train dispatchers to set up meets at sidings, force a train to wait at a siding for a priority train to pass from behind, and to keep at least one block spacing between trains going the same direction.
Timetable and train order operation was commonly used on American railroads until the 1960s, including some quite large operations such as the Wabash Railroad and the Nickel Plate Road. Train order traffic control was used in Canada until the late 1980s on the Algoma Central Railway and some spurs of the Canadian Pacific Railway.
Timetable and train order was not used widely outside North America and has been phased out in favor of radio dispatching on many light-traffic lines and electronic signals on higher-traffic lines. More detail on North American operating methods is available below.
[edit] Block signalling
If two trains cannot be running on the same section of track at the same time, then they cannot collide. So railway lines are divided into sections known as blocks, and only one train is normally allowed in each block at one time. This principle forms the basis of most railway safety systems.
[edit] History of block signalling
In the very early days of railways, on double-tracked railway lines, where trains travelled in one direction on the same stretch of track, a means was needed to space out the trains to ensure that they did not collide. In the very early days of railways, men (originally called 'policemen') were employed to stand next to the line at certain intervals with a stop watch, these men used hand signals to signal to train drivers that a preceding train had passed more or less than a certain number of minutes ago, this was called "time interval working". If a train had passed the man only a short while ago, the following train was expected to slow down or stop to allow sufficient space to develop between the trains, to prevent a collision.
This system was flawed, however, as the watchman had no way of knowing whether the preceding train had cleared the tracks ahead. And so if the preceding train broke down or stopped for some reason, the following train would have no way of knowing, and collide with it rear-on. Accidents of this type were common in the early days of railways. However, with the invention of the electrical telegraph, it became possible for the station or signal box ahead to send a message (usually a bell ring) back to confirm that a train had passed and that the line ahead was clear; this was called the "block system".
Fixed mechanical signals began to replace hand signals from the 1830s. Signals were originally worked locally, but later it became practice to operate all the signals at a particular place from levers grouped together in a signal box. When the all-clear message was received, a signalman would pull a lever which would move the signal into the 'clear' position. Signal boxes were placed at regular intervals along the line.
The block system came into use gradually during the 1850s and 1860s but became mandatory in the United Kingdom after Parliament passed legislation in 1889 as a response to numerous railway accidents, particularly Armagh. This required block signalling for passenger railways, along with interlocking and most of the practices still required and used today. Similar legislation was passed by the United States around the same period.
Not all blocks are signalled using fixed signals placed along the track. On single line railways in the UK, particularly those with low usage, it is common to use token systems that rely upon the physical possession of a unique object by a train's driver as authority to occupy the single line, normally in addition to fixed signals.
[edit] Entering and leaving a manually-controlled block
Before allowing a train to enter a block, the signalman must be certain that the block is not already occupied. And when a train leaves a block, he must communicate that fact to the signalman controlling entry to the block. Even if the signalman receives advice that the previous train has left the block, he is usually required to seek permission from the next signal box to admit the next train into the section. When a train arrives at the end of the block section, before the signalman sends the message that the train has arrived, he must sight an end-of-train marker on the back of the last vehicle. This ensures that no part of the train has become detached and left in the section. The end of train marker might be a white disc by day or a steady or flashing red lamp. If the train has entered the next block before the signalman sees that the disc or lamp is missing, he will contact the next signal box to stop the train and investigate.
[edit] Permissive block
In a permissive block system trains are permitted to pass signals indicating the line ahead is occupied, but only to do so in a manner where they can stop safely driving by sight. This allows improved efficiency in some situations and is mostly used in the USA, and in most countries is restricted to freight trains only, and may be restricted depending on the level of visibility.
Permissive block working may also be used in an emergency, either when a driver is unable to get in touch with the signalman after being held at a danger signal for a specific time, although this is only allowed when the signal does not protect any conflicting moves, and also where the signaller is unable to get in touch with the next signal box to make sure the previous train has passed, for example if the telegraph wires are down. In these cases, trains must proceed at very low speed (typically 20mph or less) and able to stop short of any obstruction, and in most cases it will not be allowed during times of poor visibility (e.g fog or falling snow).
An absolute block system is itself not entirely absolute. Multiple trains may enter a block given specific authorisation. This is necessary in order to join trains together, split trains, rescue failed trains and the like. The signalman in giving authorization also ensures the driver knows precisely what to expect ahead, and the driver must operate the train in a safe manner considering this information. Generally, the signal will remain at danger, and the driver will be given verbal authority, usually accompanied by a yellow flag, to pass the signal at danger, and the presence of the train in front will be explained. At locations where trains regularly enter occupied blocks, such as stations where coupling up takes place, a subsidiary signal, sometimes known as a "calling on" signal, will be provided for these movements; otherwise they are accomplished through train orders.
[edit] Automatic block
With automatic block signalling, signals indicate whether or not a train may enter the block based on automatic detection of whether the block is occupied or clear. The signals could also be controlled by a signalman, so that they only provide a proceed indication if both the signalman sets the signal accordingly and the block is clear.
[edit] Track circuits
One of the most common methods of determining if a block is occupied or clear is by using a track circuit to detect the presence of a train in the block. The track at either end of the block is electrically insulated, and electrical current from a battery or power supply is fed to both running rails at one end of the block. A relay at the other end is connected to both rails. When the block is unoccupied, the circuit is completed, and the relay is energized. However, when a train passes a signal and enters a block, the metal wheels and axle of the train short-circuit the current in the block, and the relay is de-energized.
This method does not explicitly need to check that the entire train has left the block. If part of the train is left in the block, that part will continue to be detected by the track circuit.
This type of circuit is used to detect trains, both for the purpose of setting the signal indication and for providing various interlocking functions — for example, not permitting points to be moved when a train is standing on them. Electrical circuits are also used to prove switch points as being in proper position before a signal over them may be cleared. Modern UK trains and staff working in track circuit block areas, carry operating clips so that, in the event of a derailment fouling an adjacent track, the derailed train's guard can operate the track circuit similar to the way a train would; this triggers danger signals on that track and can be used to prevent a collision with the derailed trains before the train's crew is able to contact the signalman.
[edit] Axle counters
An alternative method of determining the occupied status of a block is with devices at the beginning and end of the block that count the number of wheels, axles, or etc. that enter the block and that leave the block, and compare the results. If the same number leave as enter, the block is assumed to be clear.
[edit] Fixed block
Most blocks are 'fixed' blocks, i.e. they delineate a section of track between two defined points. On timetable, train order, and token-based systems, blocks usually start and end at selected stations. On signalling-based systems, blocks start and end at signals.
The lengths of blocks are designed to allow trains to operate as frequently as necessary. A lightly-used branch line might have blocks many kilometres long, whilst a busy commuter railway might have blocks a few hundred metres long.
A train is not permitted to enter a block until a signal indicates that the train may proceed, a dispatcher or signalman instructs the driver accordingly, or the driver takes possession of the applicable token. In most cases, a train cannot enter the block until not only the block itself is clear of trains, but there is also an empty section beyond the end of the block for at least the distance required to stop the train. In signalling-based systems with closely-space signals, this overlap could be up to the signal following the one at the end of the section, effectively enforcing a space between trains of two blocks.
When calculating the size of the blocks and hence the spacing between the signals, the following has to be taken into account:
- Line speed (the maximum speed the train is allowed to travel)
- Gradient (to compensate for the assistance or otherwise afforded to deceleration)
- The braking characteristics of the train(s) that travel on that line
- Sighting (the ability of the driver to see the signal)
- Reaction time (of the driver)
Historically some lines operated rules where certain large high speed trains were signalled under differing rules and only given the right of way if two blocks in front of the train were clear.
[edit] Moving block
A disadvantage of fixed blocks is that the faster trains are permitted to run, the longer the stopping distance, and therefore the longer the blocks need to be. This decreases a line's capacity.
With Moving Block, computers are used to calculate a 'safe zone' around each moving train that no other train is allowed to enter. The system depends on precise knowledge of where each train is and how fast it is moving. Each train determines its location, direction and speed by a combination of several sensors: active and passive markers along the track, trainborne tachometers and speedometers. GPS is not used for this because it is not accurate enough, and it does not work in underground tunnels. With Moving Block, lineside signals are unnecessary, and instructions are passed direct to the trains. It has the advantage of increasing track capacity by allowing trains to run much closer together while still maintaining required safety margins.
Moving Block is in use on London's Docklands Light Railway, New York's Canarsie "L" Line, and planned for future use on the Jubilee Line. It was supposed to be the enabling technology on the modernisation of Britain's West Coast Main Line which would allow the running of trains at a far higher maximum speed (140mph), but the technology was deemed not mature enough, considering the large number of junctions on the WCML, and the plan was dropped at much expense to the British taxpayer[citation needed]. It forms part of the European Rail Traffic Management System's level-3 specification for future installation in the European Train Control System, which will (at level 3) feature moving blocks that allow trains to follow each other at exact braking distance.
[edit] Fixed signals
Timetable and train order operation still has some significant flaws, such as an over-reliance on the ability of the crew of a stranded train to let other trains know of the problem, and a general intolerance for human error. When everything goes perfectly it works well, but mistakes are easy and deadly.
Timetable and train order is only suitable for railway lines which carry relatively little traffic, and is unworkable on busy rail lines because it requires great separation between trains. Where this is the case, physical signals need to be used (either mechanical semaphore signals, or - more commonly in the modern era - electric light signals) to show the train crew whether the line ahead is occupied and to ensure that sufficient space is kept between trains to allow them to stop.
[edit] Mechanical signals
The oldest forms of signal displayed their different indications by a part of the signal being physically moved. The earliest types comprised a board that was either turned face-on and fully visible to the driver, or rotated away so as to be practically invisible. While this type of signal is still in use in some countries (e.g. France and Germany), by far the most common form of mechanical signal worldwide is the semaphore signal. This comprises a pivoted arm or blade that can be inclined at different angles. The most restrictive indication is when the arm is horizontal.
For running during times of darkness, one or more lights were usually provided at each signal. Typically this comprised a permanently-lit oil lamp with movable coloured spectacles in front that would alter the colour of the light. The driver therefore had to learn one set of indications for day time viewing and another for night time viewing.
Mechanical signals were usually remotely operated by wire from a lever in a signal box, but electrical or hydraulic operation is also used nowadays, for signals that are located too far away for manual operation.
[edit] Colour light signals
On most modern railways, colour light signals have largely replaced mechanical ones. Colour light signals have the advantage of displaying the same aspects by night as by day, and require less maintenance than mechanical signals.
Although signals vary widely between countries and even railways within a given country, a typical system of aspects would be:
- Green: Proceed at line speed. Expect to find next signal displaying green or yellow.
- Yellow: Prepare to find next signal displaying red.
- Red: Stop.
On some railways, colour light signals display the same set of aspects as shown by the lights on mechanical signals during darkness.
Double signalling is sometimes used; this is the method used in some areas of New South Wales, Australia. It derives from semaphore signalling. Two sets of lights are displayed, one above the other. The upper light is the condition at the current signal, and the lower is an indication of the aspect being shown the next signal. Some lights have a small lamp at the bottom; this is an indication to continue at low speed.
The double signals indicate:
- green over green: continue
- green over yellow: attention, next signal at green over red
- green over red: caution, next signal at stop
- red over red: stop and stay stopped
- red over red with small lamp lit: low speed, 25 km/h.
[edit] Route Signalling and Speed Signalling
Signalling of British origin generally conforms to the principle of route signalling. Most railway systems around the world, however, use what is known as speed signalling.
With route signalling, the train driver is informed which route the train will take beyond each signal (except where only one route is possible). This is achieved by a route indicator attached to the signal. The driver uses his route knowledge, reinforced by speed restriction signs fixed at the lineside, to drive the train at the correct speed for the route being taken.
With speed signalling, the train driver is not informed which route the train will take, but the signal aspect tells him what speed he may proceed at. Speed signalling requires a far greater range of signal aspects than route signalling, but less dependence is placed on drivers' route knowledge.
[edit] Approach release
When the train is routed towards a diverging route that must be taken at a speed significantly less than the mainline speed, the driver must be given prior warning so as to start reducing speed early enough. With speed signalling, the signals approaching the divergence will display appropriate aspects to control the train's speed, so no 'approach release' is required.
With 'route signalling', the aspects necessary to control speed do not exist, so a system known as approach release is used. This involves holding the junction signal at a restrictive aspect (typically 'stop') so that the signals on the approach show the correct sequence of caution aspects. The train driver will start to brake in accordance with the caution aspect, without necessarily being aware that the diverging route has in fact been set. As the train approaches the junction signal, its aspect may clear to whatever aspect the current track occupancy ahead will permit.
Where the turnout speed is the same, or nearly the same, as the mainline speed, approach release is not necessary.
[edit] Safety systems
The consequence of a driver/engineer failing to respond to a signal's indication can be disastrous. As a result, various auxiliary safety systems have been devised. Any such system will necessitate the fitment of trainborne equipment to some degree. Some systems only intervene in the event of a signal being passed at danger. Others include audible and/or visual indications inside the driver's cab to supplement the lineside signals. Automatic brake application occurs if the driver should fail to acknowledge a warning. Some systems act intermittently (at each signal), but the most sophisticated systems provide continuous supervision.
In-cab safety systems are of great benefit during fog, when poor visibility would otherwise require that restrictive measures be put in place.
[edit] Cab signalling
The most recent train control systems use modern electronic systems to indicate the state of the track ahead to the driver on cab displays at all times and can halt a train automatically if a signal is passed at danger.
[edit] Interlocking
In early days, the signalman was responsible for ensuring any points (US: switches) were set correctly before allowing a train to proceed. Mistakes were made and accidents occurred, sometimes with fatalities. The concept of interlocking of points, signals, and other appliances was introduced to improve safety. Interlocking prevents the signalman from operating appliances in an unsafe sequence, such as setting the signal to 'clear' while one or more sets of points in the route ahead of the signal are improperly set.
Early interlocking systems used mechanical devices both to operate the signalling appliances and ensure their safe operation. From about the 1930s, electrical relay interlockings were used. Since the late 1980s, new interlocking systems have tended to be of the electronic variety.
[edit] References
- Railroad's traffic control systems by Frank W. Brian, Trains May 1, 2006, retrieved August 16, 2006
- General Code of Operating Rules, Fifth Edition. Copyright 2005 General Code of Operating Rules Committee (modified by BNSF Railway Company)
[edit] See also
- Automatic train protection system
- Interlocking
- List of rail diagrams
- Railroad switch
- Railway semaphores
- Railway signal
- Signal box
- Train speed optimization
Country-specific signalling:
- Australian railway signalling
- British absolute block signalling
- German railway signalling
- North American railway signaling
- Norwegian railway signalling
- Swedish railway signalling
- UK railway signalling
[edit] External links
- The Signal Page (TSP) is a website about railway signalling, world wide
- The European Railway Signalling Server
- Docklands Light Railway signalling system
- Railroad Signalling: North American Signaling and Safety
- Railroad Signaling and Communications - photos and info
- RailServe.com Signals & Communications - 100+ links
- Information on traditional British signalling
- Railways: History, Signalling, Engineering [1] [2] [3] [4] and [5]
- Railway Signalling and Operations FAQ
- North American Signaling
- Safeworking systems in Victoria (Australia)
