Automatic Warning System

150px|thumb|The AWS 'sunflower' indicator inside a [[British Rail Class 27|Class 27 showing a warning indication has been acknowledged.]]

Automatic Warning System (AWS) is a railway safety system invented 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 the train driver fails to acknowledge a warning indication, an emergency brake application is initiated by the AWS; 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.

Principles of operation

thumb|Driver's AWS equipment in a [[British Rail Class 43 (HST)|Class 43 driving cab]]

AWS is a system based on trains detecting magnetic fields. These magnetic fields are created by permanent magnets and electromagnets installed on 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.

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 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 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, 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 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.

GWR automatic train control

The first system to be put into wide use was developed in 1905 by 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 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 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 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 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 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 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 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 the third-rail principle (Smithfield Market, Paddington Suburban and Addison Road). At the entrance to the electrified sections a particular, high-profile contact ramp ( instead of the usual ) raised the locomotive's contact shoe until it engaged with a ratchet on the frame. A corresponding raised ramp at 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). It was tested by the Southern Railway, London & North Eastern Railway and the London, Midland & Scottish Railway but these trials came to nothing.

In 1947, Hudd, now working for the LMS, equipped the London, Tilbury and Southend line, a division of the LMS, with his system. Following the Nationalisation of Britain's railways and the creation of British Railways (BR) in 1948, experimental work continued. A requirement was formulated for a standardised AWS solution that would be suitable for deployment across all parts of the British railway network, operate under all weather conditions, and work with all forms of signalling and traction then in use, including on electrified lines. By mid-1959, the AWS system had been partially deployed, having become operational between Kings Cross and York, Euston and Rugby, and Edinburgh and Glasgow.

A colour light signal displaying a double yellow (steady or flashing), single yellow or red aspect
A reduction in permissible speed
A temporary or emergency speed restriction
An automatic barrier crossing locally monitored (ABCL), an automatic open crossing locally monitored (AOCL), or an open crossing (OC).

AWS was based on a 1930 system developed by Alfred Ernest Hudd and marketed as 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

Network Rail (NR) AWS consists of:

A permanent magnet set centrally between the rails and usually positioned such that it is encountered before the signal to which it relates. The top of the magnet casing is nominally level with the running surface of the rails (to within ).
A cab indicator that can show a black disk or a yellow and black "exploding" disk, known as the "AWS sunflower"
A control unit that connects the system to the brakes on the train
A driver's AWS acknowledgement button
An AWS control panel

The system works on a set/reset principle.

When the signal is at 'clear' or green ("off"), the electromagnet is energised. As 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 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 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.

thumb|BR Standard Strength AWS track equipment

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 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 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 . Below this speed, the caution horn warning will always be given, but it will be automatically cancelled when 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 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 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, 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 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 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.

thumb|BR AWS Test Shed Inductor

'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 above 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 the 'Standard Strength' magnets.

AWS is provided at most main aspect signals on running lines, though there are some exceptions: This was a recommendation of the inquiry into the derailment at Morpeth on 7 May 1969.

From 1977, a portable AWS permanent magnet was fitted ahead of the warning board on the approach to temporary speed restrictions (TSRs). This was a recommendation of the inquiry into the derailment at Nuneaton on 6 June 1975, which occurred when the driver missed a TSR warning board due to its lights being extinguished.
From 1990, AWS permanent magnets were installed immediately ahead of certain 'high risk' stop signals, as a SPAD mitigation measure. This additional AWS magnet was suppressed when the associated signal showed a 'proceed' aspect. Since the introduction of the Train Protection & Warning System (TPWS) it is no longer current practice to use AWS for this purpose. SPAD indicators were also used.

Bi-directional operation

thumb|Bidirectional AWS, the permanent magnet is in the middle and there is an electromagnet on each side of it

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.

Other countries

thumb|upright=0.5|A sign used in [[Queensland, Australia to remind drivers to check the AWS sunflower, and prepare to stop at the next signal if it is illuminated.]]

The BR AWS system is also used in:

Northern Ireland Railways
Hong Kong, MTR East Rail line (only used by intercity through trains; local trains operated by MTR Corporation use TBL as of 2012, enhanced with ATP/ATO - due to be upgraded to CBTC by 2021)
Queensland, Australia; sometimes enhanced with ATP. At the other extreme Queensland also provides a permanent magnet at the fixed distant signal of unattended crossing loops. This is also sometimes comes with AWS Signs.
Adelaide, South Australia
Taiwan Railways Administration EMU100, EMU200 series (used alongside ATS-SN/ATS-P, replaced with ATP in 2006)
Experimental French system, half mechanical and half electrical (1913)
Liberia; One of the mining railways in this country had a more advanced AWS system that employed two or three magnets of either polarity and located near the rails to avoid the suppression problem. The system was therefore able to give more aspects than the BR version.

Automatic Warning System

Principles of operation

GWR automatic train control

Network Rail

Bi-directional operation

Other countries

See also

References

Literature

Further reading