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91-512: Automatic Warning System ( AWS ) is a railway safety system invented and predominantly used in the United Kingdom. It provides a train driver with an audible indication of whether the next signal they are approaching is clear or at caution. Depending on the upcoming signal state, the AWS will either produce a 'horn' sound (as a warning indication), or a 'bell' sound (as a clear indication). If

182-474: A double track railway is normally signalled in one direction only, with all signals facing the same direction on either line. Where bidirectional signalling is installed, signals face in both directions on both tracks (sometimes known as 'reversible working' where lines are not normally used for bidirectional working). Signals are generally not provided for controlling movements within sidings or yard areas. Signals have aspects and indications . The aspect

273-433: A fail-safe is a design feature or practice that, in the event of a failure of the design feature, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safety to a particular hazard, a system being "fail-safe" does not mean that failure is naturally inconsequential, but rather that the system's design prevents or mitigates unsafe consequences of

364-421: A fail-safe mechanism, if the driver fails to press the AWS acknowledgement button for a warning indication in sufficient time, the emergency brakes will automatically apply, bringing the train to a stop. After stopping, the driver can now press the AWS acknowledgement button, and the brakes will automatically release after a safety time out period has elapsed. AWS works in the same way as for signals, except that

455-400: A traffic light . Hoods and shields are generally provided to shade the lights from sunlight which could cause false indications. Searchlight signals were the most often used signal type in the U.S. until recently. In these, a single incandescent light bulb is used in each head, and either an A.C. or D.C. relay mechanism is used to move a coloured spectacle (or "roundel") in front of

546-486: A "Stop" (or "Stop and Stay") indication, and permissive signals, which display a "Stop & Proceed" aspect. Furthermore, a permissive signal may be marked as a Grade Signal where a train does not need to physically stop for a "Stop & Proceed" signal, but only decelerate to a speed slow enough to stop short of any obstructions. Interlocking ('controlled') signals are typically absolute, while automatic signals (i.e. those controlled through track occupancy alone, not by

637-412: A battery. At each distant signal, a long ramp was placed between the rails. This ramp consisted of a straight metal blade set edge-on, almost parallel to the direction of travel (the blade was slightly offset from parallel so in its fixed position it would not wear a groove into the locomotives' contact shoes), mounted on a wooden support. As the locomotive passed over the ramp, a sprung contact shoe beneath

728-406: A bracket which itself is mounted on a post. The left hand signal then controls the left-hand track, and the right signal the right-hand track. A gantry or signal bridge may also be used. This consists of a platform extending over the tracks; the signals are mounted on this platform over the tracks they control. In some situations or places, such as in tunnels, where there is insufficient room for

819-459: A control logic which detects discrepancies. An example for this are many aircraft systems, among them inertial navigation systems and pitot tubes . During the Cold War , "failsafe point" was the term used for the point of no return for American Strategic Air Command nuclear bombers, just outside Soviet airspace. In the event of receiving an attack order, the bombers were required to linger at

910-439: A device will not endanger lives or property when it fails. Fail-secure, also called fail-closed, means that access or data will not fall into the wrong hands in a security failure. Sometimes the approaches suggest opposite solutions. For example, if a building catches fire, fail-safe systems would unlock doors to ensure quick escape and allow firefighters inside, while fail-secure would lock doors to prevent unauthorized access to

1001-413: A distinctive 'ping') and leave the 'sunflower' black. This AWS clear indication lets the driver know that the next signal is showing 'clear' and that the AWS system is working. If the next signal is displaying a restrictive aspect (e.g. caution or stop) the AWS audible indicator will sound a continuous alarm. The driver then has approximately 2 seconds to press and release the AWS acknowledgement button (if

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1092-418: A fixed magnet located at the service braking distance before the speed reduction. A single fixed magnet will always cause a warning indication to the driver, which the driver must acknowledge to prevent the emergency brake applying. A trackside warning board will also advise the driver of the speed requirement ahead. This list of limitations is not exhaustive: Early devices used a mechanical connection between

1183-402: A green light on its own, which is high speed. A lamp proving relay would detect the reduction in current when more than two lamps are not working in a failed feather indicator, and prevent the green from showing. It can also display an indication on the signaller's panel. Due to this possibility, most signals are configured to be failsafe . For example, a flashing aspect can be used to display

1274-533: A less restrictive signal. In this case, if the relay that controls the flashing fails, the signal becomes more restricting. A flashing yellow, in Canada and the United States, is part of an advance clear to stop indication, which means the second signal ahead is stop . A solid yellow means clear to stop , which means the next signal ahead is stop . Signals were originally controlled by levers situated at

1365-425: A modern railroad may have different rules governing the interpretation of signal aspects. For example, stop aspect refers to any signal aspect that does not allow the driver to pass the signal. Signals control motion past the point at which the signal stands and into the next section of track. They may also convey information about the state of the next signal to be encountered. Signals are sometimes said to "protect"

1456-492: 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. These signals had two or at most three positions. Semaphore signals were developed in France at the end of the 18th century, before being later adopted by the railways. The first railway semaphore was erected by Charles Hutton Gregory on

1547-531: A permissive signal has the lower set of lights offset (usually to the right) from the upper lights; in Victoria and New Zealand, an absolute signal displaying a red or white "A" light is also treated as a permissive signal. Some types of signal display separate permissive and absolute stop aspects. In Germany, the rules which apply to the respective signal are indicated by a vertical plate on the signal's post ( Mastschild ). Operating rules normally specify that

1638-671: A pilot on the Staten Island Railway in New York City, at the time a B&O subsidiary; they were also applied to the Chicago and Alton Railroad when the latter was under B&O control, as well as on the B&;O itself. With the disappearance of the B&O into CSX they have been gradually replaced with NORAC color light signals. Lineside signals need to be mounted in proximity to the track which they control. When

1729-422: A position. In normal use the locomotive battery was subject to constant drain holding closed the valve in the vacuum train pipe so to keep this to a minimum an automatic cut-off switch was incorporated which disconnected the battery when the locomotive was not in use and the vacuum in the train pipe had dropped away. It was possible for specially equipped GWR locomotives to operate over shared lines electrified on

1820-408: A post or gantry, signals may be mounted at ground level. Such signals may be physically smaller (termed dwarf signals ). Rapid transit systems commonly use only dwarf signals due to restricted space. In many systems, dwarf signals are only used to display 'restrictive' aspects such as low speed or shunt aspects, and do not normally indicate 'running' aspects. Occasionally, a signal may be mounted to

1911-875: A procedure is not carried out or carried out incorrectly no dangerous action results. For example: Fail-safe ( foolproof ) devices are also known as poka-yoke devices. Poka-yoke , a Japanese term, was coined by Shigeo Shingo , a quality expert. "Safe to fail" refers to civil engineering designs such as the Room for the River project in Netherlands and the Thames Estuary 2100 Plan which incorporate flexible adaptation strategies or climate change adaptation which provide for, and limit, damage, should severe events such as 500-year floods occur. Fail-safe and fail-secure are distinct concepts. Fail-safe means that

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2002-419: A second magnetic field of a certain polarity after the first permanent magnet, then the AWS displays a clear indication instead of a warning indication. The train detects the electromagnet polarity after the permanent magnet polarity. This is because the optional electromagnet is always installed after the permanent magnet (in the direction of travel). The electromagnet is connected to the green signal aspect , so

2093-409: A signal with an abnormality, such as one with an extinguished lamp or an entirely dark signal, must be interpreted as the most restrictive aspect – generally "Stop" or "Stop and Proceed". Signals differ both in the manner in which they display aspects and in the manner in which they are mounted with respect to the track. The oldest forms of signal displays their different indications by

2184-477: A signalman) are usually permissive. Drivers need to be aware of which signals are automatic. In current British practice for example, automatic signals have a white rectangular plate with a black horizontal line across it. In US practice a permissive signal typically is indicated by the presence of a number plate. In the Australian states of New South Wales, Victoria and South Australia, as well as New Zealand,

2275-452: A single track is involved, the signal is normally mounted on a post or mast which displays the arm or signal head at some height above the track, in order to allow it to be seen at a distance. The signal is normally placed on the engine driver 's side of the track. When multiple tracks are involved, or where space does not permit post mounting, other forms are found. In double track territory one may find two signals mounted side by side on

2366-401: A siren which provided an audible warning as well as slowly applying the train brakes. The driver was then expected to cancel the warning (restoring the system to its normal state) and apply the brakes under his own control - if he did not the brake valve solenoid would remain open, causing all vacuum to be lost and the brakes to be fully applied after about 15 seconds. The warning was cancelled by

2457-419: A speed within sighting distance of the stop signal. Under timetable and train order operation, the signals did not directly convey orders to the train crew. Instead, they directed the crew to pick up orders, possibly stopping to do so if the order warranted it. Signals are used to indicate one or more of the following: Signals can be placed: 'Running lines' are usually continuously signalled. Each line of

2548-455: A structure such as a retaining wall , bridge abutment, or overhead electrification support. Electric lamps for railway signals are often fitted with twin filaments , so that if one burns out, the other keeps the signal lit. A more complicated version of this, such as in the SL35 lamp, a filament changeover relay is fitted in series with the first filament, where if the first filament burns out,

2639-431: A train travelling in the opposite direction from that for which the track equipment is intended but not reset it as the electromagnet is encountered before the permanent magnet. To overcome this, a suppressor magnet may be installed in place of an ordinary permanent magnet. When energised, its suppressing coil diverts the magnetic flux from the permanent magnet so that no warning is received on the train. The suppressor magnet

2730-473: A warning indication for vehicles entering service. Due to the low speed used on such lines the size of the track equipment is reduced from that found on the operational network. 'Standard Strength' magnets are used everywhere except in DC third rail electrification areas and are painted yellow. The minimum field strength to operate the on-train equipment is 2 milliteslas (measured 125 mm [5 in] above

2821-424: A warning. The yellow and black indication persists until the next signal and serves as a reminder between signals that the driver is proceeding under caution. The one-second delay before the horn sounds allows the system to operate correctly down to speeds as low as 1 + 3 ⁄ 4  mph (2.8 km/h). Below this speed, the caution horn warning will always be given, but it will be automatically cancelled when

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2912-409: Is a secondary advantage of the system because temporary AWS equipment need only contain a permanent magnet. No electrical connection or supply is needed. In this case, the warning indication in the cab will persist until the next green signal is encountered. To verify that the on-train equipment is functioning correctly motive power depot exit lines are fitted with a 'Shed Test Inductor' that produces

3003-487: Is also used in: Railway signal Originally, signals displayed simple stop or proceed indications. As traffic density increased, this proved to be too limiting and refinements were added. One such refinement was the addition of distant signals on the approach to stop signals. The distant signal gave the driver warning that they were approaching a signal which might require a stop. This allowed for an overall increase in speed, since train drivers no longer had to drive at

3094-470: Is broken, the arm will move by gravity into the horizontal position. In the U.S., semaphores were employed as train order signals, with the purpose of indicating to engineers whether they should stop to receive a telegraphed order, and also as simply one form of block signalling. The introduction of electric light bulbs made it possible to produce colour light signals which were bright enough to be seen during daylight, starting in 1904. The signal head

3185-432: Is de-energised, the system is not reset. After the one-second delay within which the system can be reset, a horn warning is given until the driver acknowledges by pressing a plunger. If the driver fails to acknowledge the warning within 2.75 seconds, the brakes are automatically applied. If the driver does acknowledge the warning, the indicator disk changes to yellow and black, to remind the driver that they have acknowledged

3276-432: Is driving towards a signal that shows clear (green). The train runs over the AWS magnet (which is two magnets, first a permanent magnet and then an electromagnet). The electromagnet is energized. The AWS receiver detects a magnetic field in the sequence: South, North . The south pole comes from the permanent magnet, and the north pole comes from the electromagnet. This south then north sequence gives an AWS clear indication to

3367-413: Is fail-safe since loss of power will cause it to act like an ordinary permanent magnet. A cheaper alternative is the installation of a lineside sign that notifies the driver to cancel and ignore the warning. This sign is a blue square board with a white St Andrew's cross on it (or a yellow board with a black cross, if provided in conjunction with a temporary speed restriction). With mechanical signalling,

3458-425: Is included in the distant-signal control wiring to ensure the AWS "clear" indication is only given when the distant is proved "off" – mechanical semaphore distants have a contact in the electromagnet coil circuit closed only when the arm is raised or lowered by at least 27.5 degrees. Colour-light signals have a current sensing relay in the lamp lighting circuit to prove the signal alight, this is used in combination with

3549-400: Is made up of 1 permanent magnet, and an optional electromagnet. The permanent magnet is uncontrollable, and always produces a constant magnetic field of unchanging polarity. A train running over the permanent magnet will deliver an AWS warning indication to the train driver. The optional electromagnet can be used to provide the train driver with an AWS clear indication. If the train AWS detects

3640-490: Is one where the position of the lights, rather than their colour, determines the meaning. The aspect consists solely of a pattern of illuminated lights, which are all of the same colour. In many countries, small position light signals are used as shunting signals, while the main signals are of colour light form. Also, many tramway systems (such as the Metro of Wolverhampton) use position light signals. A system combining aspects of

3731-431: Is particularly useful on high speed railways . In the absence of lineside signals, fixed markers may be provided at those places where signals would otherwise exist, to mark the limit of a movement authority. Usually, signals and other equipment (such as track circuits and level crossing equipment), are powered from a low voltage supply. The specific voltage varies with the country and equipment used. The reason behind this

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3822-452: Is that the low voltage allows easy operation from storage batteries and indeed, in some parts of the world (and previously in many more locations, before the widespread adoption of electricity), batteries are the primary power source, as mains power may be unavailable at that location. In urban built-up areas, the trend is now to power signal equipment directly from mains power, with batteries only as backup. Fail-safe In engineering ,

3913-408: Is the portion of a colour light signal which displays the aspects. To display a larger number of indications, a single signal might have multiple signal heads. Some systems used a single head coupled with auxiliary lights to modify the basic aspect. Colour light signals come in two forms. The most prevalent form is the multi-unit type, with separate lights and lenses for each colour, in the manner of

4004-414: Is the visual appearance of the signal; the indication is the meaning. In American practice the indications have conventional names, so that for instance "Medium Approach" means "Proceed at not exceeding medium speed; be prepared to stop at next signal". Different railroads historically assigned different meanings to the same aspect, so it is common as a result of mergers to find that different divisions of

4095-524: The Great Western Railway (GWR) and protected by UK patents 12661 and 25955. Its benefits over previous systems were that it could be used at high speed and that it sounded a confirmation in the cab when a signal was passed at clear. In the final version of the GWR system, the locomotives were fitted with a solenoid -operated valve into the vacuum train pipe, maintained in the closed position by

4186-562: The London and Croydon Railway (later the Brighton) at New Cross Gate , southeast London, in 1841. It was similar in form to the optical telegraphs then being replaced on land by the electric telegraph . Gregory's installation was inspected and approved for the Board of Trade by Major-General Charles Pasley . Pasley had invented a system of optical telegraphy through semaphores in 1822 for

4277-626: The Strowger Automatic Telephone Exchange Company of Chicago). It was tested by the Southern Railway , London & North Eastern Railway and the London, Midland & Scottish Railway but these trials came to nothing. In 1948 Hudd, now working for the LMS, equipped the London, Tilbury and Southend line , a division of the LMS, with his system. It was successful and British Railways developed

4368-465: The "Strowger-Hudd" system. An earlier contact system, installed on the Great Western Railway since 1906 and known as automatic train control (ATC), was gradually supplanted by AWS within the Western Region of British Railways . Network Rail (NR) AWS consists of: The system works on a set/reset principle. When the signal is at 'clear' or green ("off"), the electromagnet is energised. As

4459-639: The 'Standard Strength' magnets. AWS is provided at most main aspect signals on running lines, though there are some exceptions: Because the permanent magnet is located in the centre of the track, it operates in both directions. The permanent magnet can be suppressed by an electric coil of suitable strength. Where signals applying to opposing directions of travel on the same line are suitably positioned relative to each other (i.e. facing each other and about 400yds apart), common track equipment may be used, comprising an unsuppressed permanent magnet sandwiched between with both signals' electro-magnets. The BR AWS system

4550-435: The AWS system was installed only at distant signals but, with multi-aspect signalling, it is fitted at all main line signals. All signal aspects, except green, cause the horn to sound and the indicator disc to change to yellow on black. AWS equipment without electromagnets is fitted at locations where a caution signal is invariably required or where a temporary caution is needed (for example, a temporary speed restriction). This

4641-604: The British military, and appears to have suggested to Gregory the application of the semaphore to railway signaling. The semaphore was afterwards rapidly adopted as a fixed signal nearly universally. Disc signals, such as those made by the Hall Signal Company , were sometimes used, but semaphores could be read at much longer distances. The invention of the electric light , which could be made brighter than oil lamps and hence visible both by night and day, resulted in

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4732-446: The building. The opposite of fail-closed is called fail-open . Fail active operational can be installed on systems that have a high degree of redundancy so that a single failure of any part of the system can be tolerated (fail active operational) and a second failure can be detected – at which point the system will turn itself off (uncouple, fail passive). One way of accomplishing this is to have three identical systems installed, and

4823-520: The colour and position systems was developed on the Baltimore and Ohio Railroad (B&O) in 1920 and was patented by L.F. Loree and F.P. Patenall. It is similar to the position light system with the central light removed and the resulting pairs of lights colored in correspondence to the angle they make: green for the vertical pair, amber for the right diagonal pair, and red for the horizontal pair. An additional pair, colored "lunar white", may be added on

4914-421: The company had installed it on about 100 miles of track. In 1907 Frank Wyatt Prentice patented a radio signalling system using a continuous cable laid between the rails energized by a spark generator to relay " Hertzian Waves " to the locomotive. When the electrical waves were active they caused metal filings in a coherer on the locomotive to clump together and allow a current from a battery to pass. The signal

5005-408: The development of position light signals and colour-light signals at the beginning of the 20th century, which gradually displaced semaphores. A few remain in modern operations in the United Kingdom. Mechanical signals may be operated manually, connected to a lever in a signal-box, by electric motors, or hydraulically. The signals are designed to be fail-safe so that if power is lost or a linkage

5096-643: The disadvantage of having moving parts which may be deliberately tampered with. This had led to them becoming less common during the last fifteen to twenty years when vandalism began to render them vulnerable to false indications. However, in some other countries, such as on the Italian railways ( FS ) as from the Regolamento Segnali , they are still the standard colour light signal albeit with new installations being as outlined below. More recently, clusters of LEDs have started to be used in place of

5187-457: The driver depressing a spring-laden toggle lever on the ATC apparatus in the cab; the key and circuitry was arranged so that it was the lever returning to its normal position after being depressed and not the depressing of the lever that reset the system - this was to prevent the system being overridden by drivers jamming the lever in the downward position or the lever accidentally becoming stuck in such

5278-402: The driver keeps the button held down, the AWS will not be cancelled). After pressing the AWS acknowledgement button, the AWS audible indicator is silenced and the AWS visual indicator changes to a pattern of black and yellow spokes. This yellow spoke pattern persists until the train reaches the next AWS magnet and serves as a reminder to the driver of the restrictive signal aspect they passed. As

5369-415: The driver will only receive an AWS clear indication if the signal is clear (green). The permanent magnet always produces a south pole . If the electromagnet is energized to produce a north pole, the AWS will give the driver an AWS clear indication. Multiple unit trains have an AWS receiver at each end. Vehicles that can operate singly (single car DMUs and locomotives) only have one; this could be either at

5460-401: The driver. A train is driving towards a signal that shows caution (yellow). The train runs over the AWS magnet (which is two magnets, first a permanent magnet and then an electromagnet). The electromagnet is de-energized (i.e. it is not powered). The AWS receiver detects only one magnetic field in the sequence: South . The reason only one magnetic field was detected is because the electromagnet

5551-412: The electromagnet resets the system if the driver has not already done so. The display will indicate all black once the system resets itself. The system is fail-safe since, in the event of a loss of power, only the electro-magnet is affected and therefore all trains passing will receive a warning. The system suffers one drawback in that on single track lines, the track equipment will set the AWS system on

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5642-400: The end of the electrified section released the ratchet. It was found, however, that the heavy traction current could interfere with the reliable operation of the on-board equipment when traversing these routes and it was for this reason that, in 1949, the otherwise "well proven" GWR system was not selected as the national standard (see below). Notwithstanding the heavy commitment of maintaining

5733-455: The failsafe point and wait for a second confirming order; until one was received, they would not arm their bombs or proceed further. The design was to prevent any single failure of the American command system causing nuclear war. This sense of the term entered the American popular lexicon with the publishing of the 1962 novel Fail-Safe . (Other nuclear war command control systems have used

5824-459: The front or rear depending on the direction the vehicle is traveling in. The equipment on a train consists of; The polarities in this example are relevant to the UK. The permanent magnet produces a south pole in the UK. Other countries may use permanent magnet that produces a north pole. The key operational principle is that the electromagnet produces the opposite pole of the permanent magnet. A train

5915-442: The incandescent lamps, reflectors and lenses. These use less power and have a purported working life of ten years, but this may not in reality be the case. Operating rules generally dictate that a dark signal be interpreted as giving the most restrictive indication it can display (generally "stop" or "stop and proceed"). Many colour light systems have circuitry to detect such failures in lamps or mechanism. A position light signal

6006-434: The lamp. In this manner, gravity (fail safe) returns the red roundel into the lamp's optical path. In effect, this mechanism is very similar to the colour light signal that is included in an electrically operated semaphore signal, except that the omission of the semaphore arm allows the roundels to be miniaturized and enclosed in a weatherproof housing. Widely used in the U.S. from World War II onward, searchlight signals have

6097-528: The lineside and locomotive batteries, the GWR installed the equipment on all its main lines. For many years, Western Region (successors to the GWR) locomotives were dual fitted with both GWR ATC and BR AWS system. By the 1930s, other railway companies, under pressure from the Ministry of Transport , were considering systems of their own. A non-contact method based on magnetic induction was preferred, to eliminate

6188-414: The locomotive was lifted and the battery circuit holding closed the brake valve was broken. In the case of a clear signal, current from a lineside battery energising the ramp (but at opposite polarity) passed to the locomotive through the contact and maintained the brake valve in the closed position, with the reversed-polarity current ringing a bell in the cab. To ensure that the mechanism had time to act when

6279-417: The locomotive was travelling at high speed, and the external current therefore supplied only for an instant, a "slow releasing relay" both extended the period of operation and supplemented the power from the external supply with current from the locomotive battery. Each distant signal had its own battery, operating at 12.5 V or more; the resistance if the power came directly from the controlling signal box

6370-560: The mechanism further by providing a visual indication in the cab of the aspect of the last signal passed. In 1956, the Ministry of Transport evaluated the GWR, LTS and BR systems and selected the one developed by BR as standard for Britain's railways. This was in response to the Harrow & Wealdstone accident in 1952. AWS was later extended to give warnings for; AWS was based on a 1930 system developed by Alfred Ernest Hudd and marketed as

6461-467: The mechanism – and they came to nothing. In Germany, the Kofler system used arms projecting from signal posts to engage with a pair of levers, one representing caution and the other stop , mounted on the locomotive cab roof. To address the problem of operation at speed, the sprung mounting for the levers was connected directly to the locomotive's axle box to ensure correct alignment. When Berlin's S-Bahn

6552-409: The other diagonal for restricting indications. Speed signalling is indicated not by additional signal heads, but by a system of white or amber "orbital" lights placed in one of six positions above and below the main head. The position above or below indicates the current speed, while the left-to-right position indicates the speed at the next signal (full, medium, or slow in both cases). Dwarf signals have

6643-442: The points or switches, section of track, etc. that they are ahead of. The term "ahead of" can be confusing, so official UK practice is to use the terms in rear of and in advance of . When a train is waiting at a signal it is "in rear of" that signal and the danger being protected by the signal is "in advance of" the train and signal. In North American practice, a distinction must be made between absolute signals, which can display

6734-466: The presence of trains and alter signal aspects to reflect their presence or absence. Some locomotives are equipped to display cab signals . These can display signal indications through patterns of lights in the locomotive cab, or in simple systems merely produce an audible sound to warn the driver of a restrictive aspect. Occasionally, cab signals are used by themselves, but more commonly they are used to supplement signals placed at lineside. Cab signalling

6825-568: The problems caused by snowfall and day-to-day wear of the contacts which had been discovered in existing systems. The Strowger-Hudd system of Alfred Ernest Hudd ( c.  1883  – 1958) used a pair of magnets, one a permanent magnet and one an electro-magnet, acting in sequence as the train passed over them. Hudd patented his invention and offered it for development to the Automatic Telephone Manufacturing Company of Liverpool (a subsidiary of

6916-408: The relay controlling the green aspect to energise the AWS electro-magnet. In a Solid State Interlocking the signal module has a "Green-Proved" output from its driver electronics that is used to energise the electromagnet. When the distant signal is at 'caution' or yellow (on), the electro-magnet is de-energised. As the train passes, the permanent magnet sets the system. However, since the electromagnet

7007-418: The relay drops and lights the second filament. This filament fail relay also activates an alarm in the signal box. When lamps fail, this can result in aspects that are less restrictive (high speed) than when the lamps are correctly lit. This is potentially dangerous. For example, in UK practice, if a white "feather" indicator fails, the low speed feather combined with a green light, which is low speed, becomes

7098-408: The same aspects as full-sized signals. One of the advantages claimed for the system is that burned-out bulbs produce aspects which can be interpreted unambiguously as either the intended indication (for the main head) or as a more restrictive indication (for the orbitals—if only the central head is lit, the indication is either slow or restricting). Colour position lights (CPLs) were first installed as

7189-558: The signal and the locomotive. In 1840, the locomotive engineer Edward Bury experimented with a system whereby a lever at track level, connected to the signal, sounded the locomotive's whistle and turned a cab-mounted red lamp. Ten years later, Colonel William Yolland of the Railway Inspectorate was calling for a system that not only alerted the driver but also automatically applied the brakes when signals were passed at danger but no satisfactory method of bringing this about

7280-442: The signals, and later by levers grouped together and connected to the signal by wire cables, or pipes supported on rollers (US). Often these levers were placed in a special building, known as a signal box (UK) or interlocking tower (US), and eventually they were mechanically interlocked to prevent the display of a signal contrary to the alignment of the switch points. Automatic traffic control systems added track circuits to detect

7371-643: The system's failure. If and when a "fail-safe" system fails, it remains at least as safe as it was before the failure. Since many types of failure are possible, failure mode and effects analysis is used to examine failure situations and recommend safety design and procedures. Some systems can never be made fail-safe, as continuous availability is needed. Redundancy , fault tolerance , or contingency plans are used for these situations (e.g. multiple independently controlled and fuel-fed engines). Examples include: Examples include: As well as physical devices and systems fail-safe procedures can be created so that if

7462-406: The third-rail principle ( Smithfield Market , Paddington Suburban and Addison Road ). At the entrance to the electrified sections a particular, high-profile contact ramp ( 4 + 1 ⁄ 2  in [110 mm] instead of the usual 2 + 1 ⁄ 2  in [64 mm]) raised the locomotive's contact shoe until it engaged with a ratchet on the frame. A corresponding raised ramp at

7553-428: The track equipment casing). Typical track equipment produces a field of 5 mT (measured under the same conditions). Shed Test Inductors typically produce a field of 2.5 mT (measured under the same conditions). Where DC third rail electrification is installed 'Extra Strength' magnets are fitted and are painted green. This is because the current in the third rail produces a magnetic field of its own which would swamp

7644-410: The track. The polarity and sequence of magnetic fields detected by a train determine the type of indication given to the train driver. A magnet, known as an AWS magnet is installed on the track center line. The magnetic field of the magnet is set based on the next signal aspect. The train detects the polarity of magnetic field via an AWS receiver, permanently mounted under the train. An AWS magnet

7735-454: The train driver fails to acknowledge a warning indication, an emergency brake application is initiated by the AWS. However if the driver correctly acknowledges the warning indication by pressing an acknowledgement button, then a visual 'sunflower' is displayed to the driver, as a reminder of the warning. AWS is a system based on trains detecting magnetic fields. These magnetic fields are created by permanent magnets and electromagnets installed on

7826-442: The train passes, the permanent magnet sets the system. A short time later, as the train moves forward, the electromagnet resets the system. Once so reset, a bell is sounded (a chime on newer stock) and the indicator is set to all black if it is not already so. No acknowledgement is required from the driver. The system must be reset within one second of being set, otherwise it behaves as for a warning indication. An additional safeguard

7917-484: Was electrified in 1929, a development of this system, with the contact levers moved from the roofs to the sides of the trains, was installed at the same time. The first useful device was invented by Vincent Raven of the North Eastern Railway in 1895, patent number 23384. Although this provided audible warning only, it did indicate to the driver when points ahead were set for a diverging route. By 1909,

8008-422: Was found. In 1873, United Kingdom Patent No. 3286 was granted to Charles Davidson and Charles Duffy Williams for a system in which, if a signal were passed at danger, a trackside lever operated the locomotive's whistle, applied the brake, shut off steam and alerted the guard. Numerous similar patents followed but they all bore the same disadvantage – that they could not be used at higher speeds for risk of damage to

8099-460: Was not energized. This makes the electromagnet invisible to the AWS receiver. This south pole by itself results in an AWS warning indication to the driver. As the train approaches a signal, it will pass over an AWS magnet. The AWS visual indicator ('sunflower') in the driver's cab will change to all black . If the signal being approached is displaying a 'clear' aspect, then AWS will sound a bell tone (modern trains have an electronic sounder that makes

8190-414: Was thought too great (the locomotive equipment required 500  mA ). Instead, a 3 V circuit from a switch in the signal box operated a relay in the battery box. When the signal was at 'caution' or 'danger', the ramp battery was disconnected and so could not replace the locomotive's battery current: the brake valve solenoid would then be released causing air to be admitted to the vacuum train pipe via

8281-407: Was turned off if the block were not "clear"; no current passed through the coherer and a relay turned a white or green light in the cab to red and applied the brakes. The London & South Western Railway installed the system on its Hampton Court branch line in 1911, but shortly after removed it when the line was electrified . The first system to be put into wide use was developed in 1905 by

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