Misplaced Pages

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101-508: According to the UK Rail Safety and Standards Board , the purpose of TPWS is to stop a train by automatically initiating a brake demand, where TPWS track equipment is fitted, if the train has: passed a signal at danger without authority; approached a signal at danger too fast; approached a reduction in permissible speed too fast; approached buffer stops too fast. TPWS is not designed to prevent signals passed at danger (SPADs) but to mitigate

202-450: A Stop semaphore, and so exhibited only a yellow light at night. Where a signal consists of a combination of a stop and distant arms a mechanism is included to prevent the distant arm clearing while the stop arm is at ‘danger’. Current British practice mandates that semaphore signals, both upper and lower quadrant types, are inclined at an angle of 45 degrees from horizontal to display an "off" indication. In total, colour-light signals in

303-480: A brake application on a train about to pass a signal at danger regardless of speed. In a standard installation there are two pairs of loops, colloquially referred to as "grids" or "toast racks". Both pairs consist of an 'arming' and a 'trigger' loop. If the signal is at danger the loops will be energised. If the signal is clear, the loops will de-energise. The first pair, the Overspeed Sensor System (OSS),

404-406: A caution signal, and after 10 minutes, the line was assumed to be clear. Therefore, if a train failed within a section (as was very common in the early days), the policeman controlling entry to the section would not know, and could easily give a 'clear' signal to a following train when the section was not in fact clear. The number of collisions which resulted from this, as well as the introduction of

505-554: A driver at a distance between 250–800 metres (270–870 yd) from the signal, with no obstructions within 40 metres (44 yd) and at a height of 2.5 to 3 metres (8.2 to 9.8 ft) above the left-hand rail. Signals are positioned 900–2,100 millimetres (35–83 in) from the inside edge of the left-hand rail. Right-hand signals are used in situations where local conditions make a left-hand placement unsuitable. Ground mounted signals are rarely so critical for alignment (an advantage of ground mounting) and are often used in tunnels, where

606-484: A false intervention. As a temporary solution, drivers were instructed to pass the buffer stop OSSs at 5 mph, eliminating the problem, but meaning that trains no longer had the momentum to roll to the normal stopping point and requiring drivers to apply power beyond the OSS, just a short distance from the buffers, arguably making a buffer stop collision more likely than before TPWS was fitted. The redesigned 'mini loops', roughly

707-478: A heavy counterweight with push-pull rod between counterweight and arm linkage (generally assisted by the "spectacle" that carries the coloured lenses for use at night) to do that, while upper-quadrant signals fall back to "danger" under the weight of the arm. During the 1870s, almost all the British railway companies standardised on the use of semaphore signals, which were then invariably of the lower quadrant type. From

808-533: A not-for-profit entity, its primary purpose being to bring about improved health and safety performance throughout Britain's railway network. In the fulfilment of this purpose, the Board undertakes numerous safety-focused monitoring and continuous improvement programmes, such as the railway supplier quality assurance scheme RISQS and the Confidential Incident Reporting & Analysis System (CIRAS). It

909-478: A number of limitations, and that while it provided a relatively cheap stop-gap prior to the widescale introduction of ATP and ERTMS, nothing should impede the installation of the much more capable European Train Control System . A pair of electronic loops are placed 50–450 metres on the approach side of the signal, energized when it is at danger. The distance between the loops determines the minimum speed at which

1010-419: A problem as it is considered that a steady double-yellow followed by a flashing single-yellow aspect sequence is acceptable. However, safety circuitry is connected to the single-yellow flashing supply to ensure that a failure of the single-yellow to change over to the flashing supply would abort the "approach release from yellow" sequence and re-impose the normal "approach control from red" sequence as failure of

1111-406: A red aspect. If it is lit, the signal is "on", and if the red light is unlit, the signal is "off". The traditional British signal is the semaphore , comprising a mechanical arm that rises or drops to indicate 'clear' (termed an "upper-quadrant" or "lower-quadrant" signal, respectively). Both types are fail-safe in the event of breakage of the operating pull-wire but lower-quadrant signals require

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1212-534: A red, yellow, or green filter to show the respective aspect. The filter assembly is moved by an electro-magnet. For a double-yellow aspect a second lamp is fitted, illuminated only when required. A few traditional searchlight signals (i.e. with moving filter glasses inside) remain in use in the Clacton area. These fell out of favor by the 1960s, being replaced by the multi-lens vertically arranged signals, with searchlight style signals only being allowed in circumstances where

1313-571: A second transmitter may be placed on the approach to the signal that applies the brakes on trains going too quickly to stop at the signal, positioned to stop trains approaching at up to 75 mph (120 km/h). At around 400 high-risk locations, TPWS+ is installed with a third transmitter further in rear of the signal increasing the effectiveness to 100 mph (160 km/h). When installed in conjunction with signal controls such as 'double blocking' (i.e. two red signal aspects in succession), TPWS can be fully effective at any realistic speed. TPWS

1414-446: A series of between 2 and 5 stop signal arms on one bracket or gantry, known as splitting signals . Each arm (usually) has its own post ("doll") on the bracket, and each arm applies to one possible route. The relative heights of the posts usually convey some information about the lines to which they apply, although there is no definite standard. In some cases, the tallest post applies to the highest-speed route; in others, it applies to what

1515-421: A signal arm in the horizontal position, and "off" means a signal raised upwards or lowered downwards from pivot point (at up to 60°). With regard to newer colour-light signals, "on" is synonymous with the most restrictive aspect, while all other aspects are considered to be "off". A way to remember this is to refer to the state of the red light, or yellow light if the signal is a distant and incapable of displaying

1616-440: A stand at it, and then the driver must be warned verbally by the signaller that the line is not clear the whole distance to the next signal, then once the signaller is satisfied the driver has understood the warning, they will typically pull off the signal very slowly – the driver understands from this that they are being accepted into the occupied length of line under "Warning" Regulation 4. In colour light power box operated areas,

1717-492: A third the length of the standard ones, eliminate this problem, although due to the low speed and low margin, buffer stop OSSs are still a major cause of TPWS trips. Recent applications in the UK have, in conjunction with advanced SPAD protection techniques, used TPWS with outer home signals that protect converging junctions with a higher than average risk by controlling the speed of an approaching train an extra signal section in rear of

1818-456: A train to enter the section and clear a set of points in order to carry out a shunting move. The warning signal is the most unusual of the three types of British permissive signal. It is, like the shunt-ahead, placed under the signal governing entry to the section ahead, but its function is very different. For a signaller to accept a train, both their block section and the line for a quarter mile inside their outer home signal must usually be clear;

1919-438: Is a TPWS system fault, or will show a steady illumination if the "Temporary Isolation Switch" has been activated. There is also a separate TPWS Temporary Isolation Switch located out of reach of the driver's desk. This is operated by the driver when the train is being worked in degraded conditions such as Temporary Block Working where multiple signals need to be passed at danger with the signalman's authority. Temporarily isolating

2020-457: Is a terminal station platform starting signal. An OSS on its own may be used to protect a permanent speed restriction, or buffer stop . Although loops are standard, buffer stops may be fitted with 'mini loops', due to the very low approach speed, usually 10 mph. When buffer stops were originally fitted with TPWS using standard loops there were many instances of false applications, causing delays whilst it reset, with trains potentially blocking

2121-595: Is also works with operators to identify and address risks, and is responsible for the updating and issuing of the British Railway Rule Book , amongst its other activities. The majority of Britain's train operators are obliged to be members of the RSSB, and to ensure compliance with their membership obligations across all of their operations. During the 2010s, the RSSB was restructured in order to better fulfil its purpose. The Rail Safety and Standards Board (RSSB) which

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2222-426: Is based on the block principle whereby only one train is allowed in a signal section, there are situations when another train must enter the section, and permissive signals are used to control that movement. There are three types of permissive semaphore: calling-on, shunt-ahead, and warning signals. Today, all three look broadly the same; they are shaped like a normal semaphore stop signal, though only about two-thirds of

2323-447: Is by far the most common of the three types of subsidiary signal. It is mounted under the stop signal governing entry to (usually) a platform and, when pulled off, allows the driver to proceed cautiously for as far as the line is clear (or to the next stop signal). This can allow three basic moves to take place; The shunt-ahead signal is normally mounted under the signal governing entry to the section ahead, and, as its name implies, allows

2424-466: Is cleared for a shunting movement, the TSS loops are de-energised, but the OSS loops remain active. Where trains are signalled in opposite directions on an individual line it could be possible for an unwarranted TPWS intervention to occur as a train travelled between an OSS arming and either trigger loops that were in fact associated with different signals. To cater for this situation one signal would be nominated

2525-510: Is financed primarily via levies sourced from its members. The ORR itself supervises the RSSB, performing periodic independent reviews of the organisation for such purposes. Operationally, the RSSB has routinely employed various techniques to assess and manage risk, particularly during incident investigations, to help drive improvements in the rail industry's health and safety policies. It operates multiple teams of inspectors that act not only to ensure legal compliance and appropriate certification

2626-407: Is frequent. Because there is no margin for braking error, the warning arrangement cannot usually be applied to passenger trains: its commonest use is to allow a goods train to run into a section to shunt a siding in the middle of that section, while a train is still occupying the station ahead. Position light signals allow a train to move into a section under caution, the line ahead may be occupied so

2727-417: Is mostly used on busy routes to allow shorter headways, and on fast routes to provide longer braking distances. A flashing single or double yellow aspect indicates that a train is to take a diverging route ahead with a lower line speed than the main route. A flashing double yellow (only used in 4-aspect signalling) means that the next signal is showing flashing single yellow. A flashing single yellow means that

2828-611: Is not the same as train stops which accomplish a similar task using electro-mechanical technology. Buffer stop protection using train stops is known as ‘ Moorgate protection ' or 'Moorgate control’. TPWS was developed by British Rail and its successor Railtrack , following a determination in 1994 that British Rail's Automatic Train Protection system was not economical, costing £600,000,000 equivalent to £979,431,929 in 2019 to implement, compared to value in lives saved: £3-£4 million (4,897,160 - 6,529,546 in 2019), per life saved, which

2929-417: Is obtained and maintained by the organisations involved, but to foster a climate of continuous improvement throughout all elements of the rail industry. To this end, the RSSB has been committed to the continued development of various models, tools, and competences to support its members. New models and tools have also been developed through the stewardship of the RSSB. RSSB is also responsible for operating

3030-494: Is restricted to railways with a maximum speed limit of up to 125 miles per hour (201 km/h). This is the maximum speed at which the train can travel safely using line-side signalling; if the train runs any faster, it will not be possible for the train driver to safely read colour-light signalling. Trains operating at speeds faster than 125 mph (for example on High Speed 1 ) use an in- cab signalling system that automatically determines and calculates speed restrictions. In

3131-400: Is sited at a position determined by line speed and gradient. The loops are separated by a distance that should not be traversed within less than a pre-determined period of time of about one second if the train is running at a safe speed approaching the signal at danger. The exact timings are 974 milliseconds for passenger trains and 1218 milliseconds for freight trains, determined by equipment on

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3232-419: Is usually the lowest of the principal aspects. This places the most restrictive aspect nearest to the driver's eyeline and also reduces the possibility of the lens becoming obscured by snow building up on the lens hood of an aspect below. Similarly, on ground-mounted signals the most restrictive aspect is positioned as the highest of the principal aspects; this again places the most restrictive aspect nearest to

3333-586: The Mumbai Suburban Railway in India, on the Western Line and Central Line . Rail Safety and Standards Board The Rail Safety and Standards Board ( RSSB ) is a British independent company limited by guarantee . Interested parties include various rail industry organisations, including Network Rail , train operating companies (TOCs), and rolling stock companies (ROSCOs). The RSSB operates as

3434-551: The Trackoff programme promoting rail safety within schools, as well as the Sustainable Rail Programme. Under the supervision and direction of the RSSB, various safety-related changes have been introduced upon Britain's railways. One example is new standards for high-visibility clothing , which have been credited with a measurable improvement in the effective safety levels for rail workers and other staff interacting with

3535-402: The electrical telegraph , led to the gradual introduction of the absolute block principle; all systems of working other than this (including time-interval and permissive block) were outlawed on passenger lines in 1889, and all passenger lines were suitably equipped by 1895. As train speeds increased, it became increasingly difficult for enginemen to see hand signals given by the policemen, so

3636-520: The slick conditions produced wheel slide and the train therefore was not brought to a stop prior to the collision point. (ATP would not have prevented this circumstance either). Critics, such as those representing victims of the Ladbroke Grove and Southhall rail crashes, and ASLEF and RMT rail unions pushed for the abandonment of TPWS in the late 1990s in favor of continuing with British Rail's ATP project. A 2000 study, Automatic Train Protection for

3737-632: The "home" signal where "warning" arrangements are in force has a time release similar to approach control from red but the control is more stringent – the signal only clears when the speed of the train is detected to be less than 10 mph and only clearance to single yellow is allowed – this is called delayed yellow operation, and is often found at the approach to large stations where two trains may use one platform. Subsidiary signals are those which usually control only shunting moves, as opposed to train movements. Under this category come permissive signals and shunting signals. Although British railway operation

3838-459: The 1920s onwards, upper quadrant semaphores almost totally supplanted lower quadrant signals in Great Britain, except on former GWR lines and their succession to BR(WR) and latterly Network Rail Western Zone. There are two main types of semaphore; stop and distant. The stop signal consists of a red, square-ended arm, with a vertical white stripe typically 9-12 inches (230–300 mm) from

3939-529: The Limit of Shunt may also be signalled by a simple white floodlit board on which the words "Limit of Shunt" are written in red. British railway signalling is unusual in that it uses route signalling rather than the speed signalling used by most railways in continental Europe or North America. A driver is informed of which route they will take at a junction, rather than the speed at which they should travel through it. In semaphore areas, junctions are signalled using

4040-522: The ORR, shall help drive improvements in rail safety standards while also delivering a more efficient and sustainable railway. According to the RSSB's chief operating officer Johnny Schute, a greater level of engagement between the Board and the membership is a major priority, and that industry players typically ought to be more proactive and vocal in matters of safety. UK railway signalling#Subsidiary signals The railway signalling system used across

4141-530: The TPWS TSS loops for approximately 20 seconds (generally for passenger trains) or 60 seconds (generally for slower accelerating freight trains) or until the loops have been passed, whichever is sooner. The AWS system and the TPWS system are inter-linked and if either of these has initiated a brake application, the "Brake Demand" indicator lamp will flash. The "Temporary Isolation/Fault" indicator lamp will flash if there

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4242-588: The TPWS does not affect the AWS. The driver must reinstate the TPWS immediately at the point where normal working is resumed. As a safety feature, if they forget to do this, the TPWS will be reinstated on the next occasion that the driver's desk is shut down and then opened up again. An alternative to using derailers in Depot Personnel Protection Systems is to equip the system with TPWS. This equipment safeguards staff from unauthorised movements by using

4343-462: The TPWS equipment. Any unplanned movement will cause the train to automatically come to a stand when it has passed the relevant signal set at danger. This has the added benefit of preventing damage to the infrastructure and traction and rolling stock that a derailer system can cause. The first known installation of such a system is at Ilford Depot. TPWS equipped depot protection systems are suitable only for locations where vehicles are driven in and out of

4444-438: The TPWS panel, so that the train can pass the signal without triggering the TPWS to apply the brakes. The driver must then proceed at a speed which enables them to stop within the distance that they can see to be clear. Even if it appears that the section is clear to the next signal, they must still exercise caution. TPWS failed to prevent the 2021 Salisbury rail crash , because although the train went to full emergency braking,

4545-462: The UK display seven aspects. These are: Additionally, on the 140 miles per hour (230 km/h) trial section of line between Peterborough and York: The green aspect and the four yellow aspects are known as 'proceed aspects', as they allow the train to pass the signal; the red aspect requires the train to stop . Two-aspect systems use red and green only. Three-aspect systems include yellow. Four-aspect signalling, which also includes double yellow,

4646-410: The actions of drivers, train speed, and the use of TPWS. There are many reasons why a driver might be required to pass a signal at danger with authority. The signaller will advise the driver to pass the signal at danger, proceed with caution, be prepared to stop short of any obstruction, and then obey all other signals. Immediately before moving, the driver will press the "Trainstop Override" button on

4747-481: The agency's prime purpose is to lead the various other entities associated with Great Britain's rail network, including the ORR, rail infrastructure owner and maintainer Network Rail , and the Rail Delivery Group operating body. Amongst its responsibilities, the RSSB is responsible for the publication and updating of the British Railway Rule Book , which defines technical standards and operating procedures upon

4848-416: The conditions ahead. Typically for low speed junctions (e.g. 25 mph (40 km/h) crossover on a 90 mph (140 km/h) line), the train will be brought down to nearly standing at the signal before it clears. Approach control is achieved by maintaining the signal at danger until the approach track circuit has been occupied for a specified period of time. After the track circuit has been occupied for

4949-424: The consequences of a SPAD, by preventing a train that has had a SPAD from reaching a conflict point after the signal. A standard installation consists of an on-track transmitter adjacent to a signal, activated when the signal is at danger. A train that passes the signal will have its emergency brake activated. If the train is travelling at speed, this may be too late to stop it before the point of collision, therefore

5050-535: The days of the first British railways, "policemen" were employed by every railway company. Their jobs were many and varied, but one of their key roles was the giving of hand signals to inform engine drivers as to the state of the line ahead. They had no means of communication with their colleagues along the line, and trains were only protected by a time interval; after a train had passed them, a policeman would stop any following train if it arrived within (say) 5 minutes; for any between 5 and 10 minutes after, they would show

5151-551: The decision to cancel ATP and replace it with TPWS and the actual roll out of TPWS that Ladbroke Grove and Southall rail crash both occurred, accidents that were ATP preventable, and occurred on the Great Western line, which had been outfitted with ATP as part of the pilot studies in the early-90s. The TPWS system is used in: Since 1996, an older variant of TPWS, called the Auxiliary Warning System, has been used by

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5252-478: The driver can see it from their desk. There are two types of panel; the original 'standard' type, and a more recent 'enhanced' version, which gives separate indications for a brake demand caused by a SPAD, Overspeed or AWS. The standard type consists of two circular indicator lamps and a square push button. The push switch marked "Train Stop Override" is used to pass a signal at danger with authority . It ignores

5353-459: The driver may pass the signal with caution. When not cleared these signals are unlit, and the train driver obeys the main aspect signal. Ground position light signals (GPLS), are always illuminated and are located either near the ground or on a post with no corresponding main signal. They can display the following aspects: Shunt ahead signals are fitted with either two yellow lights, or one white and one yellow light. They are usually found at

5454-421: The driver must drive at a speed that enables them to stop short of any obstruction. Modern position lights consist of three lenses in a triangular formation. Associated position light signals (APLS) are attached to a main aspect signal and are only illuminated when a shunting movement is permitted. When the main signal aspect is red, the position light displays two white lights at an angle of 45° indicating that

5555-488: The driver to proceed but be prepared to stop short of any obstruction. When unlit, the driver obeys the main signal aspect. They can therefore function either as calling-on or shunt-ahead signals, depending on their location (the Warning Arrangement in colour-light areas, uses the main aspect in a similar fashion to approach release junction signalling, in this case it is called a Delayed Yellow ). The calling-on signal

5656-419: The driver's eyeline and reduces the possibility of obscuration through snow build up. In two-aspect signals the green aspect is typically the uppermost and the red aspect the lowest. In three-aspect signals the order, from top to bottom, is typically green-yellow-red. In four-aspect signals the order is typically yellow-green-yellow-red. The top yellow is only used in the display of the double yellow aspect and

5757-401: The earlier searchlight signals, these LED signals use one aperture to display red, yellow, and green aspects, and a second aperture to display the top yellow of a double yellow where required in four-aspect signalling areas. When junction indicators are fitted, they are typically placed above the principal aspects of a signal. Signal positioning guidance aims for a signal to be understood by

5858-427: The end, and advises the driver whether the line immediately ahead is clear or not. A stop signal must not be passed in the horizontal "on" (danger) position, except where specially authorised by the signaller's instruction. By night, it shows a red light when "on" and a green light when "off" (clear). The green light is usually produced through the use of a blue spectacle lens, which produces green when lit from behind by

5959-446: The exits of marshaling yards and sidings, and can be passed at danger for a movement in the direction for which the signal cannot be cleared (e.g. into a headshunt rather than onto the main line). This arrangement removes the requirement for the signal to be cleared every time a shunt is to take place within the sidings without fouling the main running lines. When cleared they display two white lights at 45-degrees and permit movements onto

6060-400: The frequencies for the loops, and prove the loops are intact. They interface with the signalling system. SIM Modules are colour coded red ND TSS Modules are colour coded green OD TSS Modules are colour coded brown ND OSS Modules are colour coded yellow OD OSS Modules are colour coded blue Every traction unit is fitted with a: If the loops are energised, an aerial on the underside of

6161-408: The impact, and reduce or eliminate fatalities, by at least slowing the train down. However, it is likely that in those cases the driver would have applied the emergency brakes well before the overspeed sensor. While it has been noted that there have been very few fatalities since the fitting of TPWS that would have been prevented had ATP been fitted instead. This overlooks that during the delay between

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6262-402: The introduction of solid state interlocking resulted in more stringent criteria for the use of flashing aspects in earlier installations. If a signal is to malfunction and not show any aspect, the driver if required to treat the blank signal as if it was red and contact the signaller. A failure of the changeover relay to switch on the flashing indication to the double-yellow aspect would not be

6363-431: The junction, the red signal beyond may 'step up' to a less restrictive aspect depending on the state of the line ahead. The two yellows in a flashing double-yellow flash in unison rather than alternately, but the flashing double-yellow and single-yellows are not synchronised. Flashing yellow signalling contains an additional safety vital relay typically referred to as Flashing Lamp Proving Relay (FECR) – this changes over

6464-462: The junction. If this fails the resultant TPWS application of brakes will stop the train before the point of conflict is reached. This system is referred to as TPWS OS (Outer Signal). Standard TPWS installations can only bring a train to a stop prior to passing a red signal, at 74 miles per hour (119 km/h). In 2001, it was observed that roughly one-third of the UK railway allows for a speed above 75 miles per hour (121 km/h). Further this assumes

6565-415: The lower yellow is used for the display of both the double yellow and single yellow aspects. Regulations require that a space be present between two yellow lamps for displaying the double yellow aspect. Searchlight signals were utilized from the early days of colour light signals, along side early vertically arranged signal heads in early 1920s. These have a single lamp in front of which is placed either

6666-414: The lowest position may reduce risk of obscuring of that lens by heavy snow or ice. There are standard arrangements of the lights, however unusual variations, such as horizontal mounting is allowed when demanded by local conditions or geography, such as in tunnels, areas of limited clearance, or the presence of bridges over the railway. On pole- and gantry-mounted railway signals the most restrictive aspect

6767-474: The main interlocking, hence the additional delay in proving that the junction indicator is lit prior to clearing the main signal. In the Absolute Block Signalling System, the signalling regulations provide for trains to be signalled into a section of line where the designated "overlap" past the signal is not clear – the signaller keeps the signal concerned at danger until the train has come to

6868-504: The main line. Limit of Shunt A limit of shunt signal. consists of two permanently lit red lights in a horizontal arrangement, meaning 'Stop'. No train is allowed to pass this signal as the direction will be against the normal direction of travel. A limit of shunt signal is permanently lit and cannot display any other aspect; there is no lens fitted in the proceed position on these signals. The mechanical equivalents of these shunting signals are found as miniature semaphores (the arms are

6969-514: The maintenance building from a leading driving cab - they are not suitable for use with loose coaching stock or wagon maintenance, where vehicle movements are undertaken by a propelling shunting loco (in this case the lead vehicles would not be equipped with the relevant TPWS safety equipment), nor will it prevent a run-away vehicle from entering a protected work area. Certain signals may have multiple OSSes fitted. Alternatively, usually due to low line speeds, an OSS may not be fitted. An example of this

7070-524: The majority of the United Kingdom rail network uses lineside signals to control the movement and speed of trains. The modern-day system mostly uses two, three, and four aspect colour-light signals using track circuit – or axle counter – block signalling. It is a development of the original absolute block signalling that is still being used on many secondary lines. The use of lineside signals in Britain

7171-432: The mutual obligations between the Board and its member organisations can be better fulfilled. These alterations to business practices have included a greater emphasis on transparency, the clarification of its core functions, improvements to communications, and a new focus on being proactive in its undertakings. It is reportedly intended that these changes, which were performed primarily in response to recommendations issued by

7272-496: The network. The specifications for Rail Industry Standard RIS-3279-TOM (fluorescent orange) high-visibility clothing, suitable for use on railways in the United Kingdom , are published by the RSSB. In late 2019, the RSSB announced that it was formulating a safety strategy to allow hydrogen-powered trains to be routinely operated upon the mainline rail network. During the 2010s, the RSSB has undergone structural changes so that

7373-399: The next signal at the junction is showing (steady) single yellow with an indication for a diverging route, and the signal beyond the junction is at danger (red). This sequence of increasingly restrictive aspects forces the driver to slow the train down in preparation for stopping at the red signal, and this ensures that the train crosses the junction at the appropriate speed. As the train nears

7474-425: The next stop signal. When "off", a distant signal tells the driver that all the following stop signals of the signal box are also "off", and when "on" tells the driver that one or more of these signals is likely to be at danger. By night, it shows a yellow light when "on" and a green light when "off". On many branch lines and short block sections, a distant signal was often fixed at 'Caution', standalone or mounted below

7575-452: The on board equipment will apply the train's emergency brake . When the train's TPWS receiver passes over the first loop a timer begins to count down. If the second loop is passed before the timer has reached zero, the TPWS will activate. The greater the line speed, the more widely spaced the two loops will be. There is another pair of loops at the signal, also energised when the signal is at danger. These are end to end, and thus will initiate

7676-415: The on-train equipment detects both frequencies together after having detected the arming frequency alone. Thus, an energised TSS is effective at any speed, but only if a train passes it in the right direction. Since a train may be required to pass a signal at danger during failure etc., the driver has the option to override a TSS, but not an OSS. When a subsidiary signal associated with a main aspect signal

7777-458: The one at Purley , where a driver repeatedly cancelled the AWS warning without applying the brakes, passing the danger signal at high speed. Purley was one of several high profile SPAD crashes in the late 1980s, that led to the initial plan in the 1990s for the mass rollout of ATP, that was subsequently canceled in 1994 to be replaced by TPWS. Supporters of TPWS claim that even where it could not prevent accidents due to SPADs, it would likely reduce

7878-416: The quarter mile is a precaution in case the driver fails to stop in time for the outer home signal. However, it is possible to accept a train under the "Warning Arrangement" if the block section, but not the quarter-mile overlap, is clear. As its name implies, the signaller must stop and caution the driver of the train concerned, and the warning signal simply replaces the signaller's caution where this operation

7979-469: The rail network in Britain remarked that TPWS was "in terms of avoiding “ATP preventable accidents” it is about 70% effective.", highlighting the speed limitation. That 2000 study did still conclude that TPWS was good solution for the short term of 10–15 years, but stressed that European Train Control system was the long term solution. Notably, the combination of TPWS and AWS is least effective in accidents like

8080-435: The railway considered the most important route. Traditionally, splitting distant signals would be provided – a series of side-by-side distant signals telling the driver which post on the following stop signal was off; but practice since the 1920s has erred towards providing just one distant which is locked at caution if a large speed reduction is necessary. Drivers of trains must know which signal arm applies to which route, and

8181-467: The railway supplier assurance scheme RISQS , as well as the confidential incident reporting line, Confidential Incident Reporting & Analysis System (CIRAS). It also supporting several cross-industry groups for the purpose of addressing key safety concerns, and encouraged collaboration to address such concerns. Since its establishment, the RSSB has sought to foster greater levels of customer engagement to better go about its mission. The RSSB promotes

8282-430: The railway. The RSSB's ownership is divided amongst a range of different organisations in the British railway sector, including Network Rail , infrastructure managers, train operating companies , and rolling stock companies . As a condition of their licence obligations, the majority of Britain's train operators are required by the ORR to be members of the RSSB and to comply with the obligations of membership. The RSSB

8383-521: The railways provided various types of fixed signals to do the job, operated by the policemen, or signalmen as they soon became known (it is due to this that British railway slang still names signalmen as "Bobbies"). Many types were devised, but the most successful was the semaphore, introduced in 1841 and soon becoming widespread, although some other types did linger on until the 1890s. The terms "on" and "off" are used in describing British railway signals. When describing an older semaphore, "on" refers to

8484-419: The red band is angled. Both display small red or green lights by night. There are also semaphore and disc equivalents of the yellow light shunting signals; the small-arm semaphores being painted yellow with a black stripe and the discs either black or white with a yellow stripe; by night, they show small yellow lights when "on" and small green lights when "off". Finally, instead of fixed position light signals,

8585-463: The relative luminosity of the aspects is much higher. At certain locations such as the final signal on approach to a terminus station or at a diverging route requiring a large speed reduction, approach release may be used. The driver will be "checked down" with a normal signalling sequence (green, double yellow, yellow for a four-aspect area) and the red signal clears when it is proven that the approaching train must have slowed to an appropriate speed for

8686-413: The same size as those of permissive signals) and disc varieties (the disc is about 12 inches/30 cm diameter). The small-arm semaphores are painted in the same way as a full-size stop signal, while the discs are painted white with a red horizontal band. A small-arm semaphore shows "clear" in the same way as a full-size stop signal, while a disc rotates through 45 degrees or so when pulled off so that

8787-478: The signal from the energised Train Stop System loops, and the brake will be applied to stop the train within the overlap . Multiple unit trains have an aerial at each end. Vehicles that can operate singly (single car DMUs and locomotives) only have one aerial. This would be either at the front or rear of it depending on the direction the vehicle was moving in. Every driving cab has a TPWS control panel, located where

8888-405: The signal lens would allow better signal sighting due to a physical obstructions to sight lines. By 1991, the use of searchlight signals in any future installation was prohibited. The concept had a renaissance in the 2000s with the advent of LEDs which allow the same aperture to be used to display multiple colours, while eliminating moving parts that could fail on searchlight signals. Similar to

8989-461: The signal, when cleared for the divergence for Walsall-bound trains, shows the junction-indicator with a red aspect for 2–3 seconds before the main aspect clears – this is whilst the interlocking proves sufficient elements of the junction-indicator are lit before clearing the main aspect. With route relay interlocking the proving circuitry for the junction indicator is housed locally. With a solid state or computerised signalling this proof has to pass to

9090-399: The single yellow to flash following a flashing double-yellow is considered potentially very dangerous. The design considerations determining the familiar arrangement of roadway 'traffic signals' , with red at the top, do not apply to the railway. In particular, there is no risk that a signal will be masked by a tall vehicle in front of the driver. Furthermore, to position the red aspect at

9191-406: The size, and are painted red with a white horizontal band running centrally along them. When "on", they show a small red or white light, and when "off", they display a small green light and an illuminated 'C', 'S' or 'W', depending on their function. Modern-day colour-light permissive signals consist of two white lights at 45°, normally unlit. When lit, with the main aspect showing red, they instruct

9292-452: The specified period of time, the signal is allowed to "step-up" to the highest available aspect and display the junction indicator where applicable. The length of time required varies on the design of the installation. Where a junction indicator is used an additional safety precaution ensures that failure of the indicator does not cause an irregular or mutilated display to appear. This can be observed in practice – at Bescot Stadium northbound

9393-411: The station throat, plus the risk of passengers standing to alight being thrown over by the sudden braking. This problem arose when a train passed over the arming loop so slowly that it was still detected by the train's receiver after the on-board timer had completed its cycle. The timer would reset and begin timing again, and the trigger loop then being detected within this second timing cycle would lead to

9494-539: The supply for the yellow signal transformers at each signal where flashing aspects are provided from a steady 110Va.c. to a "flashing" supply switched on and off at about 1.2 Hz or 70–72 c.p.m., once the junction points have been set, locked and detected correctly for the lower–speed divergence. This supply has to be specially provided, either from the power–box or control centre, or by a specially designed signal control module in more modern LED installations. The increased complexity in providing flashing aspects prior to

9595-408: The train picks up the radio frequency signal and passes it to the receiver. A timer measures how long it takes to pass between the arming and trigger loops. This time is used to check the speed, and if it is higher than the TPWS 'set speed', an emergency brake application is initiated. If the train is travelling slower than the TPWS set speed, but then passes the signal at danger, the aerial will receive

9696-505: The train's brakes is capable of providing a brake force of 12%g. A number of train types, most notably, the HSTs were not capable of achieving this, despite having a top speed of 125 miles per hour (201 km/h). TPWS-A was capable of stopping a train up to 100 miles per hour (160 km/h). TPWS has no ability to regulate speed after a train passes a signal at danger with authority . However, on those occasions there are strict rules governing

9797-457: The train. Freight trains use the 1.25 times longer timing because of their different braking characteristics. The first, 'arming', loop emits a frequency of 64.25  kHz . The second, 'trigger', loop has a frequency of 65.25 kHz. The other pair of loops is back to back at the signal, and is called a Train Stop System (TSS). The 'arming' and 'trigger' loops work at 66.25 kHz and 65.25 kHz respectively. The brakes will be applied if

9898-412: The yellowish flame from a paraffin lamp. The other type is the distant signal, which has a yellow arm with a 'V' ("fishtail") notch cut out of the end and a black chevron typically 9-12 inches (230–300 mm) from the end. Its purpose is to advise the driver of the state of the following stop signal(s); it may be passed in the "on" position, but the driver must slow their train to be able to stop at

9999-456: The ‘normal direction’ and fitted with ‘ND’ equipment. The other signal would be nominated the ‘opposite direction’ and fitted with ‘OD’ equipment. Opposite direction TPWS transmission frequencies are slightly different, working at 64.75 (OSS arming), 66.75 (TSS arming), and 65.75 kHz (common trigger). At the lineside there are two modules associated with each set of loops: a Signal Interface Module (SIM) and an OSS or TSS module. These generate

10100-527: Was established during 2003 as a result of a direct recommendation of the public inquiry into the Ladbroke Grove rail crash . According to the Office of Rail and Road (ORR) regulatory body: " The RSSB's principal objective is to lead and facilitate the rail industry's work to achieve continuous improvement in the health and safety performance of the railways in Great Britain. " In accordance with this principle,

10201-658: Was estimated to be 2.9 per year. Trial installations of track side and train mounted equipment were made in 1997, with trials and development continuing over the next two years. The rollout of TPWS accelerated when the Railway Safety Regulations 1999 came into force in 2003, requiring the installation of train stops at a number of types of location. However, in March 2001 the Joint Inquiry Into Train Protection Systems report found that TPWS had

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