Timetable operation
The simplest form of operation, at least in terms of equipment, is to run the system according to a timetable. Every train crew understands and adheres to a fixed schedule. Trains may only run on each track section at a scheduled time, during which they have 'possession' and no other train may use the same section. When trains run in opposite directions on a single-track railway, meeting points ("meets") are scheduled, at which each train must wait for the other at a passing place. Neither train is permitted to move before the other has arrived. In the US, the display of two green flags (green lights at night) is an indication that another train is following the first and the waiting train must wait for the next train to pass. In addition, the train carrying the flags gives eight blasts on the whistle as it approaches. The waiting train must return eight blasts before the flag carrying train may proceed. The timetable system has several disadvantages. First, there is no positive confirmation that the track ahead is clear, only that it is scheduled to be clear. The system does not allow for engine failures and other such problems, but the timetable is set up so that there should be sufficient time between trains for the crew of a failed or delayed train to walk far enough to set warning flags, flares, and ''Timetable and train order
With the advent of the telegraph in 1841, a more sophisticated system became possible because this provided a means whereby messages could be transmitted ahead of the trains. The telegraph allows the dissemination of any timetable changes, known as ''Block signalling
Trains cannot collide with each other if they are not permitted to occupy the same section of track at the same time, so railway lines are divided into sections known as ''blocks''. In normal circumstances, only one train is permitted in each block at a time. This principle forms the basis of most railway safety systems. Blocks can either be fixed (block limits are fixed along the line) or moving blocks (ends of blocks defined relative to moving trains).History of block signalling
On double tracked railway lines, which enabled trains to travel in one direction on each track, it was necessary to space trains far enough apart to ensure that they could not collide. In the very early days of railways, men (originally called 'policemen', which is the origin of UK signalmen being referred to as "bob", "bobby" or "officer", when train-crew are speaking to them via a signal telephone) were employed to stand at intervals ("blocks") along the line with aEntering and leaving a manually controlled block
Before allowing a train to enter a block, a signalman must be certain that it is not already occupied. When a train leaves a block, he must inform the signalman controlling entry to the block. Even if the signalman receives advice that the previous train has left a block, he is usually required to seek permission from the next signal box to admit the next train. When a train arrives at the end of a block section, before the signalman sends the message that the train has arrived, he must be able to see the end-of-train marker on the back of the last vehicle. This ensures that no part of the train has become detached and remains within the section. The end of train marker might be a coloured disc (usually red) by day or a coloured oil or electric lamp (again, usually red). If a train enters the next block before the signalman sees that the disc or lamp is missing, he asks the next signal box to stop the train and investigate.Permissive and absolute blocks
Under a permissive block system, trains are permitted to pass signals indicating the line ahead is occupied, but only at such a speed that they can stop safely should an obstacle come into view. This allows improved efficiency in some situations and is mostly used in the USA. In most countries it is restricted to freight trains only, and it 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 contact a signalman after being held at a danger signal for a specific time, although this is only permitted when the signal does not protect any conflicting moves, and also when the signalman is unable to contact 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 or less) so that they are able to stop short of any obstruction. In most cases, this is not allowed during times of poor visibility (e.g., fog or falling snow). Even with an absolute block system, multiple trains may enter a block with authorization. This may be necessary in order to split or join trains together, or to rescue failed trains. In giving authorization, the signalman also ensures that the driver knows precisely what to expect ahead. The driver must operate the train in a safe manner taking this information into account. Generally, the signal remains at danger, and the driver is given verbal authority, usually by a yellow flag, to pass a signal at danger, and the presence of the train in front is explained. Where trains regularly enter occupied blocks, such as stations where coupling takes place, a subsidiary signal, sometimes known as a "calling on" signal, is provided for these movements, otherwise they are accomplished through train orders.Automatic block
Under automatic block signalling, signals indicate whether or not a train may enter a block based on automatic train detection indicating whether a block is clear. The signals may also be controlled by a signalman, so that they only provide a ''proceed'' indication if the signalman sets the signal accordingly and the block is clear.Fixed block
Most blocks are "fixed", i.e. they include the section of track between two fixed 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 line might have blocks many kilometres long, but a busy commuter line 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 appropriate 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 spaced signals, this overlap could be as far as 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 therefore the spacing between the signals, the following have to be taken into account: *Line speed (the maximum permitted speed over the line-section) *Train speed (the maximum speed of different types of traffic) *Gradient (to compensate for longer or shorter braking distances) *The braking characteristics of trains (different types of train, e.g., freight, high-speed passenger, have different inertial figures) *Sighting (how far ahead a driver can see a signal) *Reaction time (of the driver) Historically, some lines operated so that certain large or high speed trains were signalled under different rules and only given the right of way if two blocks in front of the train were clear.Moving block
Under a moving block system, computers calculate a safe zone around each moving train that no other train is allowed to enter. The system depends on knowledge of the precise location and speed and direction of each train, which is determined by a combination of several sensors such as radio frequency identification along the track, ultra-wideband, radar, inertial measurement units, accelerometers and trainborne speedometers (Centralized traffic control
Centralized traffic control (CTC) is a form of railway signalling that originated in North America. CTC consolidates 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 in portions of the rail system designated as CTC territory.Train detection
Train detection refers to the presence or absence of trains on a defined section of line.Track circuits
The most common way to determine whether a section of line is occupied is by use of a track circuit. The rails at either end of each section are electrically isolated from the next section, and an electrical current is fed to both running rails at one end. A relay at the other end is connected to both rails. When the section is unoccupied, the relay coil completes an electrical circuit, and is energized. However, when a train enters the section, it short-circuits the current in the rails, and the relay is de-energized. This method does not explicitly need to check that the entire train has left the section. If part of the train remains in the section, the track circuit detects that part. This type of circuit detects the absence of trains, both for setting the signal indication and for providing various interlocking functions—for example, preventing points from being moved while a train is approaching them. Electrical circuits also ''prove'' that points are locked in the appropriate position before the signal protecting that route can be cleared. UK trains and staff working in track circuit block areas carry track circuit operating clips (TCOC) so that, in the event of something fouling an adjacent running-line, the track circuit can be short-circuited. This places the signal protecting that line to 'danger' to stop an approaching train before the signaller can be alerted.Axle counters
An alternate method of determining the occupied status of a block uses devices located at its beginning and end that count the number of axles that enter and leave the block section. If the number of axles leaving the block section equals those that entered it, the block is assumed to be clear. Axle counters provide similar functions to track circuits, but also exhibit a few other characteristics. In a damp environment an axle counted section can be far longer than a track circuited one. The low ballast resistance of very long track circuits reduces their sensitivity. Track circuits can automatically detect some types of track defect such as a broken rail. In the event of power restoration after a power failure, an axle counted section is left in an undetermined state until a train has passed through the affected section. A track circuited section immediately detects the presence of a train in section.Fixed signals
On most railways, physicalMechanical signals
Older forms of signal displayed their different aspects by their physical position. The earliest types comprised a board that was either turned face-on and fully visible to the driver, or rotated 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. A horizontal arm is the most restrictive indication (for 'danger', 'caution', 'stop and proceed' or 'stop and stay' depending on the type of signal). To enable trains to run at night, one or more lights are usually provided at each signal. Typically this comprises a permanently lit oil lamp with movable coloured spectacles in front that alter the colour of the light. The driver therefore had to learn one set of indications for daytime viewing and another for nighttime viewing. Whilst it is normal to associate the presentation of a green light with a safe condition, this was not historically the case. In the very early days of railway signalling, the first coloured lights (associated with the turned signals above) presented a white light for 'clear' and a red light for 'danger'. Green was originally used to indicate 'caution' but fell out of use when the time interval system was discontinued. A green light subsequently replaced white for 'clear', to address concerns that a broken red lens could be taken by a driver as a false 'clear' indication. It was not until scientists at Corning Glassworks perfected a shade of yellow without any tinges of green or red that yellow became the accepted colour for 'caution'. Mechanical signals are usually remotely operated by wire from a lever in a signal box, but electrical or hydraulic operation is normally used for signals that are located too distant for manual operation.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 between 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.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''. Note: Generally both Route and Speed signalling follow the exact same rules on straight sections of track without junctions, the differences between the two system arise when there are junctions involved, as both systems have different methods of notifying trains about junctions. Under route signalling, a driver is informed which route the train will take beyond each signal (unless 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 to be taken. This method has the disadvantage that the driver may be unfamiliar with a route onto which he has been diverted due to some emergency condition. Several accidents have been caused by this alone. For this reason, in the UK drivers are only allowed to drive on routes that they have been trained on and must regularly travel over the lesser used diversionary routes to keep their route knowledge up to date. Under speed signalling, the signal aspect informs the driver at what speed he may proceed, but not necessarily the route the train will take. Speed signalling requires a far greater range of signal aspects than route signalling, but less dependence is placed on drivers' route knowledge. Many systems have come to use elements of both systems to give drivers as much information as possible. This can mean that speed signalling systems may use route indications in conjunction with speed aspects to better inform drivers of their route; for example, route indications may be used at major stations to indicate to arriving trains to which platform they are routed. Likewise, some route signalling systems indicate approach speed using theatre displays so that drivers know what speed they must travel.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 adequate prior warning. Under ''route signalling'', the aspects necessary to control speed do not exist, so a system known as ''approach release'' is often employed. 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 driver brakes 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 permits. Where the turnout speed is the same, or nearly the same, as the mainline speed, approach release is unnecessary. Under ''speed signalling'', the signals approaching the divergence display aspects appropriate to control the trains speed, so no ''approach release'' is required. There is also a system of ''flashing yellows'' used in the UK that allows trains to approach a diverging route at higher speed. This informs the driver that the route ahead is set onto a diverging line. With the advent of faster modern day trains and junctions a better system for advising drivers was required and so the following system was developed way back in the early 1980s. The system has been refined over the years, now being used internationally and it is also used on lower speed 3-aspect signalling systems where the ''single flashing yellow'' is the driver's first indication. On the 4-aspect system, if the route through the junction is clear the junction signal will display a single ''steady yellow'' aspect together with an illuminated junction indicator showing the selected route. The signal prior to the junction signal will now show a ''single flashing yellow'' aspect and the signal prior to that one will display ''two flashing yellow'' aspects. The driver's route knowledge tells them permissible speed across the diverging junction, and they will begin to slow the train upon seeing the ''two flashing yellows''. The flashing signals tell the driver that the route through the junction is set and is clear, but that beyond that the first signal on the diverging route is ''red'' so they must be prepared to stop there. As the train approaches the junction signal, the signal may ''step up'' to a less restrictive aspect (single ''yellow'', ''two yellows'' or ''green'') depending on how far ahead the line is clear.Speed-controlled approach
Some systems in the world use mechanical speed control systems in conjunction with signalling to ensure the speed of a train is limited to a specific value, in order to ensure the train is travelling at a speed in which it is able to stop before an obstruction. These systems most often use mechanical train stop devices (a small arm coming up from the rails that will apply the brakes of a train when run over) to "trip" the brakes of a train that is traveling too fast. Normally, once a train reaches a certain point on the tracks, it sets off a timer, when the timer runs out the train stop arm will lower, allowing a train to go past uninterrupted. The timing is designed so that if the train is traveling at the intended speed (or slower) then the train will be able to continue without issue, but if the train is traveling too fast, then the Train Stop will trip the train and bring it to a halt. This system can be used to ensure a train is traveling at a certain speed, which allows designers to be confident that shorter signal overlaps will be sufficient, and thus employment of this system can help to greatly improve capacity of a railway line. The system is most often used on approach to dead end junctions to stop trains from crashing into the buffers at the end, as has happened in places such asSafety systems
A train driver failing to respond to a signal's indication can be catastrophic. As a result, various auxiliary safety systems have been devised. Any such system requires installation of some degree of trainborne and wayside equipment. Some systems only intervene in the event of a signal being passed at danger (SPAD). 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. The most advanced train control systems have no driver at all relying on computers to drive the system entirely such as Skytrain in Vancouver, Canada and the metro system in Doha, Qatar. In-cab safety systems are of great benefit during fog, when poor visibility would otherwise require that restrictive measures be put in place. Safety systems are also important in urban rail where it is impossible to see around corners in subway and metro tunnels. On-board and wayside computers can track trains around tight corners at higher speeds ensuring safety.Cab signalling
Cab signalling is a subsystem that communicates signalling information into the train cab such as driving position, speed and failure alarms. Cab signaling units are important human factors engineering subsystems in modern train signalling systems. If there is an active cab, the orientation of the train is decided, i.e. the side of the active cab is considered as the front of the train. In modern systems, aInterlocking
In the early days of the railways, signalmen were responsible for ensuring any points (US: switches) were set correctly before allowing a train to proceed. Mistakes, however, led to accidents, sometimes with fatalities. The concept of the mechanicalOperating rules
Operating rules, policies and procedures are used by railroads to enhance safety. Specific operating rules may differ from country to country, and there may even be differences between separate railroads within the same country.Argentina
The Argentinian operating rules are described in the ''Reglamento interno técnico de operaciones .I.T.O.' (''technical operating rule-book).''Australia
The application of operating rules in Australia is called ''Safeworking''. The ''method of working'' for any particular region or location is referred-to as the "Safeworking system" for that region. Operating rules differ between states, although attempts are being made to formulate a national standard.North America
In North America, and especially theUnited Kingdom
The operating rulebook for the United Kingdom is called the "GE/RT8000 Rule Book", more commonly known simply as "The Rule Book" by railway employees. It is controlled by theItaly
InIndia
The Indian operating rules, called "The General Rules", are common for all zonal railways ofJapan
Japanese railway signalling was initially based on the British railway signalling system. However, as signalling has advanced to meet the requirements of the modern railway network (and as a result of US influence), the Japanese signalling system is a mixture of British route signalling and American speed signalling.See also
*Notes
General references
* * * * Director of S&T Engineering, West Midlands Project GrouExternal links