Railway signalling

From Free net encyclopedia

Image:Mergefrom.gif It has been suggested that Railway signal be merged into this article or section. ([[{{{2|: talk:Railway_signalling}}}|Discuss]])
Image:SR signals from cab of standard five.jpg

Railway signalling is a safety system used on railways 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.

In Australia, railway signalling is known as safeworking, perhaps due to the fact that not all safeworking systems involve signals.

Most forms of train control involve messages being passed from those in charge of the rail network or portions of it to the train crew; these are known as 'signals' and from this the topic of train control is known as 'signalling'.

Contents

[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 most 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.

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.

Timetable and train order was not used widely outside North America. A form of train orders, called Track Warrant Control or Direct Traffic Control, is still used on many light-traffic lines. In this method, orders are dictated by a dispatcher to a train crew via radio or mobile phone. The train crew uses a fill-in-the-blank form to record the dispatcher's orders.

[edit]

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]

Blocks

If two trains cannot be running on the same section of track at the same time, then they cannot collide. This notion forms the basis of most signalling systems.

The rail network is divided into sections, known as blocks. Two trains are not allowed to be in the same block at the same time. A train cannot enter a block until it is permitted, generally by a signal that the block ahead is empty.

On high-speed railways, block signalling has disadvantages because the required block length to safely stop a train would severely decrease the line's capacity. Also, signals become increasingly hard to spot and recognize at higher speeds. Several cab signalling systems have been developed to overcome those disadvantages. The European Train Control System will (at level 3) feature moving blocks that allow trains to follow each other at exact braking distance. 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.

There are two distinct forms of block signalling. Absolute block signalling (as is required by UK law since 1889 for all passenger lines in the UK) is operated in a manner designed to ensure two trains may not occupy the same block at once. Telegraph codes are used to communicate between signal boxes, each of which controls a block. The signalman only allows entry to a given block when no train occupies the block. When the train traverses the block the signalman signals ahead to the next block who will accept the train if they have space or delay it otherwise. As an additional safety check all trains have a tail lamp. If no tail lamp is seen the signalman assumes his block is not empty after the train has passed as the lack of a tail lamp may indicate the train has come apart. Instead the block remains occupied and the signalman telegraphs the next signalbox to halt the train and investigate.

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.

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.

[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".

Mechanical semaphore signals replaced hand signals in the early 1840s. When the all-clear message was received, a signalman in a signal box would pull a lever which would move the signal into the all-clear position. This required the placing of signal boxes 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. 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, 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.

[edit]

Safety systems

The purpose of signalling is to inform the driver when it is safe to proceed on the line ahead. 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 points in the route the signal governs are improperly set. Early interlocking systems used mechanical devices both to operate the signalling appliances and ensure their safe operation, but contemporary interlocking systems perform using complex electronic circuitry.

A second area of safety concern was fog. Because of the propensity for heavy fog in some parts of the British Isles, fog signal rules were established on the UK railway system to keep train traffic moving without incurring the severe delays that would be necessary if drivers had to stop or travel slowly up to each signal and read its indication. During heavy fog, fogsignalmen would be stationed at distant signals with a lantern and detonators — small explosive charges that could be strapped to the rail to be exploded by the wheels of a train. The fogsignalman's duty was to repeat the indication of the signal using his lantern; the semaphore blade was usually obscured by fog and hence invisible to the driver of a moving train. If the distant signal were displaying Caution (warning that the home signal was at danger), the detonators remained on the rail and the fogsignalman would show a yellow lamp to show Caution; if the distant signal were clear, the detonators would be removed from the rails and a green lamp would be displayed.

In the UK all trains were required to carry detonators so that in the event of an incident (such as an emergency brake application) the train crew could place them on the line to warn any other train approaching from the rear or on any adjacent tracks.

Britain's Great Western Railway introduced the Automatic Train Control (ATC) system in 1906. This system is the forerunner of today's Automatic Warning System (AWS) and consists of an electrical system that sounded a bell in the cab as the train approached a signal at clear. Power was fed through a metal ramp to a pickup on the underside of the locomotive to power the bell. An absence of the electrical voltage on the ramp caused a warning horn to sound in the locomotive's cab. The driver then had a set time to acknowledge the warning and start braking his train accordingly. If the driver did not acknowledge the warning, the brakes would be applied automatically.

The current system of AWS in use on Britain's railways is similar in principle to the Great Western's ATC but does not rely on physical contact between a ramp and the train; instead an inductive system is used. The most recent train control systems use modern electronic systems to indicate the state of the signals ahead to the driver on cab displays at all times and can halt a train automatically if a signal is passed at danger.

The railroads of the United States use systems of different designs; some systems are similar in behavior to AWS wherein they provide warnings only in conjunction with fixed signal locations. Other systems provide a continuous in-cab signal indication.

One of the basic components of most signalling systems is the track circuit. Briefly, electrical current from a battery 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, the presence of a train or rolling stock in the block creates a short circuit between battery and relay, and the relay is de-energized.

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 carry electrical cables so that, in the event of a derailment fouling an adjacent track, the derailed train's guard can short-circuit the two rails of the adjacent track; 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]

Coloured light signals

On most modern railways, detection of the train's position on the line and signal changing are done automatically, and coloured-light signals have largely replaced mechanical ones.

The light signals usually mean:

  • Green: Proceed at line speed. Expect to find next signal green or yellow.
  • Yellow: Prepare for next signal to be at red.
  • Red: Stop.

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)

The track at either end of the block is electrically insulated, and within the block a small electrical current passes through the track. When a train passes a signal and enters a block, the metal wheels and axle of the train short-circuit the current, which causes a relay associated with the track circuit to itself become de-energized.

When the relay is de-energized, the signal which the train has just passed automatically turns from green (or yellow) to red, the signal behind that one automatically turns yellow, and the signals behind that one can show green.

If any train is following behind, the yellow signal will warn it to slow down in order to stop at the next signal. If, however, the train in front has passed into the next block, the following train will come across another yellow signal. If the train in front is travelling faster than the following train and clears two blocks, the following train will come across a green signal.

The genius of this relay-based track circuit system is its fail-safe mode of operation. If the relay accidentally becomes de-energized, it fails, or the circuit is otherwise disturbed, the track circuit's status and the potential for presence of a train are unknown. In all of these cases, the track circuit causes the associated signal to drop to red.

In the UK a variation of this is used whereby each set of signal lights has four lights in order from top down: yellow, green, yellow, red. The red and green signals are used as described, as is the lower yellow light. The upper yellow light is used to provide a 'double yellow' signal which serves as a warning that the next block is on yellow, thus providing a warning of a red signal a further block in advance.

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: caution, 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.

The track circuit is also used by signalmen to detect exactly where a train is on a line. In a signal box, a map of the stretch of track the signal box is controlling is usually put up on the wall (called a mimic panel), and when a train enters a particular block, a light representing that block on the map lights up to show the train's location. In the UK, in large signalboxes it is usually numbered with the train's headcode so that the signalman can see which train it is. Track circuits are also used to trigger automatic barriers (gates) on level crossings (grade crossings in U.S. usage).

[edit]

Notes on U.S. signalling

U.S. railways used a far greater variety of signalling systems than other countries. There have never been national standards for signal appearance and operation, so each of the hundreds of rail lines developed its own signalling techniques. Many major railroads have adopted the Northeast Operating Rules Advisory Committee (NORAC) rules, which provide some standards for signals. However, due to different legacy systems still in use, certain single signal indications can be shown in over a dozen ways. Further, there is a good deal of bidirectional (duplex) track in the U.S., and also a fair amount of "dark track" with no signalling equipment installed.

[edit]

Moving block signalling

This is a system which uses computers to calculate a 'safe zone' behind each moving train, which no other train may enter. It depends on precise knowledge of where each train is and how fast it is moving. 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.

Moving Block is in use on London's Docklands Light Railway and planned for future use on the Jubilee Line. It was supposed to be the enabling technolgy 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 and the plan was dropped at much expense to the British taxpayer. It forms part of the European Rail Traffic Management System's level-3 specification for future European installation.

[edit]

See also

[edit]

External links

Retrieved from "http://www.netipedia.com/index.php/Railway_signalling"