Editing Automatic Warning System (section)

== History ==

=== Early devices ===
[[File:Zugsicherung sbahn berlin.jpg|thumb|Berlin S-Bahn [[train stop]] in its engaged (left) and disengaged (right) position]]

Early devices used a mechanical connection between the signal and the locomotive. In 1840, the locomotive engineer [[Bury, Curtis, and Kennedy|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 [[HM Railway Inspectorate|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 was found.<ref>Vanns (1997) p.129</ref>

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.<ref>{{cite news |title=The Inventor |work=[[The English Mechanic and World of Science]] |issue=448 |date=24 October 1873}}</ref> 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 [[Journal bearing|axle box]] to ensure correct alignment.<ref>{{cite patent |country= US|number=1885061 |status= |title= Automatic safety appliance for railway vehicles against overrunning of signals |pubdate= 25 October 1932|gdate= |fdate= 28 August 1931|pridate= |inventor= George Kofler|invent1= |invent2= |assign1= |assign2= |class= |url=}}</ref> 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.{{fact|date=January 2020}}

The first useful device was invented by [[Vincent Raven]] of the [[North Eastern Railway (United Kingdom)|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-gap transmitter|spark generator]] to relay "[[Heinrich Hertz#Electromagnetic research|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 [[Signalling block system|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.<ref>{{cite patent |country=US |number=843550 |status= |title= Electric Signaling System|pubdate=5 February 1907 |gdate= |fdate= |pridate= |inventor= Frank Wyatt Prentice|invent1= |invent2= |assign1= |assign2= |class= |url=}}</ref> 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 [[LSWR suburban lines#Electrification of the LSWR network|electrified]].<ref>{{cite book|last=Jackson|first=Alan A|title=London's Local Railways|date=1978|publisher=[[David & Charles]]|location=Newton Abbot, England|isbn=0-7153-7479-6|page=99}}</ref>

=== 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&nbsp;V or more; the [[Electrical resistance and conductance|resistance]] if the power came directly from the controlling signal box was thought too great (the locomotive equipment required 500&nbsp;[[milliamp|mA]]). Instead, a 3&nbsp;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.<ref name=awjd>{{cite journal |last1=Dymond |first1=A. W. J. |title=The Automatic Train Control System of the Great Western Railway |journal=Transactions |date=10 March 1936 |issue=206 |pages=102;115|publisher=G. W. R. Swindon Engineering Society}}</ref>

It was possible for specially equipped GWR locomotives to operate over shared lines [[Railway electrification in Great Britain|electrified]] on the third-rail principle ([[Smithfield Market]], [[Paddington tube station (Circle and Hammersmith & City lines)#Metropolitan Railway|Paddington Suburban]] and [[Kensington (Olympia) railway station|Addison Road]]). At the entrance to the electrified sections a particular, high-profile contact ramp ({{cvt|4+1/2|in|disp=sqbr}} instead of the usual {{cvt|2+1/2|in|disp=sqbr}}) 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).<ref name=awjd/><ref name=grsw/>

Notwithstanding the heavy commitment of maintaining the lineside and locomotive batteries, the GWR installed the equipment on all its main lines. For many years, [[Western Region of British Railways|Western Region]] (successors to the GWR) locomotives were dual fitted with both GWR ATC and BR AWS system.

=== Strowger–Hudd system ===
By the 1930s, other railway companies, under pressure from the [[Department for Transport|Ministry of Transport]], were considering systems of their own. A non-contact method based on [[Magnetic field|magnetic induction]] was preferred, to eliminate 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}}{{Snd}}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 Electric#England|Automatic Telephone Manufacturing Company]] of Liverpool (a subsidiary of the [[Automatic Electric|Strowger Automatic Telephone Exchange Company]] of Chicago, Illinois).<ref name=GG/><ref>{{cite patent |country=US|number= 1599470|status= |title= Railway Signalling System|pubdate= |gdate= 14 September 1926|fdate= |pridate= |inventor= Alfred Ernest Hudd|invent1= |invent2= |assign1= |assign2= |class= |url=}}</ref> It was tested by the [[Southern Railway (England)|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 Rail]]ways developed 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 rail crash|Harrow & Wealdstone accident]] in 1952.<ref name=grsw>{{cite book|last1=Wilson|first1=G R S|title=Report on the Double Collision which occurred on 8th October, 1952, at Harrow and Wealdstone Station in the London Midland Region British Railways|date=12 June 1953|publisher=[[Office of Public Sector Information|HM Stationery Office]]|location=London|pages=25–29|oclc=24689450}}</ref>

=== Additional functions ===
AWS was later extended to give warnings for;<ref name="Awsrules 1.4">{{cite web| title =AWS & TPWS Handbook: Section 1.4 "AWS indications and their meanings"| url =https://www.rssb.co.uk/rgs/rulebooks/RS522%20Iss%203.pdf| publisher =RSSB| access-date =2018-09-24| archive-date =5 December 2016| archive-url =https://web.archive.org/web/20161205171951/http://rssb.co.uk/rgs/rulebooks/RS522| url-status =dead}}</ref>

* A [[UK railway signalling#Running signals|colour light signal]] displaying a double yellow (steady or flashing), single yellow or red aspect
* A [[UK railway signalling#Speed indicators|reduction in permissible speed]]
* A [[UK railway signalling#Speed restrictions|temporary or emergency speed restriction]]
* An [[Level crossings in the United Kingdom#Types of crossing|automatic barrier crossing locally monitored]] (ABCL), an [[Level crossings in the United Kingdom#Types of crossing|automatic open crossing locally monitored]] (AOCL), or an [[Level crossings in the United Kingdom#Types of crossing|open crossing]] (OC).
AWS was based on a 1930 system developed by Alfred Ernest Hudd<ref name=GG>{{Cite web|url=https://www.gracesguide.co.uk/Alfred_Ernest_Hudd|title=Alfred Ernest Hudd - Graces Guide|website=www.gracesguide.co.uk|language=en|access-date=2019-08-06}}</ref> 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#United Kingdom|automatic train control]] (ATC), was gradually supplanted by AWS within the [[Western Region of British Railways]].