Editing Automatic Warning System (section)

=== 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>