Before PJW's comments above were posted, I had asked yet another Peter who is far more learned in these matters than me to give some information. Here is his response
Quote:In order to answer the question fully, it might be useful to understand how and why AWS developed. In summary, it was invented in Reading by Amos Brooker although patented by Insell and Jacobs. It was introduced in 1906 as the ‘Audible Signal’ with the intention that it should replace the Distant signal. The double line example was installed on the Henley branch, and the single line on the Fairford branch. It used a hump-backed ramp that could have a voltage applied to it, placed between the rails some 200 yards on the approach side of the Distant signal. If the Distant were ‘on’, the ramp was electrically dead and would cause a brake application on a locomotive unless the warning were acknowledged. Thus an un-acknowledged alarm would bring the train to a stand within the ‘allowed’ distance – between the Distant signal and a pre-determined place beyond the Home signal known as the clearing point. If the Distant were ‘off’ the ramp would be electrically live and cause a bell to ring in the locomotive cab, telling the driver where the distant signal was and that all the stop signals to which it applied were clear.
Under the GWR this system was known as ‘Automatic Train Control’. After Nationalisation the contact-less magnetic Hudd version which had been trialled by the LMS on the Fenchurch Street to Shoeburyness line was developed into the standard system, with the ex-Western Region ATC installations being replaced during the 1970s.
Fitting of AWS to semaphore signalling is to signals that display a ‘caution’ aspect only, since (unless all Stop signals to which a Distant signal applies are clear) the signaller is responsible for clearing stop signals only when (s)he is satisfied that a train is under control. Additionally, the spacing of the Stop signals is not prescribed in the same way as for Track Circuit Block (TCB) Multiple Aspect Signalling (MAS). This remains true when semaphore Stop signals are replaced by Red/Green colour light signals. AWS is not fitted to a Red/Yellow/Green signal within a semaphore area where the yellow aspect is only displayed as a Delayed Yellow (when not all the Stop signals ahead of it are clear), since under those circumstances the train will have been brought under control from the Distant signal in rear (which is fitted with AWS). Finally, a Red/Yellow/Green signal in a semaphore area where the Yellow aspect is a free yellow with the track section proved clear up to and including an overlap, would be fitted with AWS since that signal is acting as a ‘Distant’.
Turning to MAS, a major decision point was reached on the ex-WR when Gloucester panel signal G272 was introduced at Standish Junction. This was in the days before TPWS; G272 was the first Red/Green M.A.S. signal and the question was should it be fitted with AWS? The decision was to fit it, and using today’s language it was because it was ‘more ALARP’ to fit than not, given that a driver was to receive an AWS indication at every other main signal on Gloucester panel. It was not strictly required to be fitted since G272 does not display a caution aspect. Since then, all Red/Green signals in TCB MAS areas have been fitted with AWS for consistency.