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Shunt, Calling On and Warning Signals ?

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Shunt, Calling On and Warning Signals ?

Looking at some Central Wales Signal Box Diagrams recently, there are a couple of places where the notation refers to a 'Warning Signal', the signal symbol being the same for any of the three with 'S', 'C' or 'W' written alongside.
I get the impression the 'W', warning signal is a LMS period description, though the other two are also in use, post grouping.
I'm locked into a LNWR pre-grouping mentality, and there's no clear definition of a 'W' signal in R.Fosters LNWR Signalling book.
I believe I can understand the use of 'Calling On' and 'Shunt', but why or what is a 'Warning' signal, over and above the other two?

Edited by Penlan

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There were three categories of subsidiary signal (I'll largely ignore Draw Ahead because it gets complicated for little real purpose, alas) -


1. Shunt Ahead.  Always placed below the Section Signal (i.e. the  stop signal controlling entrance into the block section in advance).  It gave authority for a shunting movement to draw into the section for as far as was necessary to carry out the shunt section when there wasn't sufficient room to carry out the shunt within Station Limits.  Not to be confused with a 'shunting signal'.


2. Calling On.   Placed below a stop signal to give authority to allow a second train or shunt movement to enter a signal section which was already occupied,  In some cases and particularly following a 1930s Rule Book revision it was used solely to give authority to enter an occupied block section while the signal giving authority to enter an occupied signal section in Station Limits was retitled  'Draw Ahead Signal' (although it looked more or less like a Calling On Signal).  That nonsense was changed back in the 1950 Rule Book so a Calling On signal, which uncovers a letter 'C' when off,  went back to applying to any signal section - i.e. the length of line between any two stop signals, which of course includes a block section as it too is a section of line between two consecutive stop signals.


3. Warning.  Placed below a section signal to indicate to a Driver that their train has been accepted in accordance with Block Regulation 5, the Warning Acceptance which means that the line is only clear to the Home signal at the next signal box in advance and the full 440 yard Clearing Point is already occupied (also known as 'Station or Junction Blocked').  A Driver being allowed into a section 'under the Warning' must regulate the passage of his train ready to stop at the Home Signal of the next 'box in advance.  (There is another use which I will avoid here because it was uncommon and rather complicated plus it was not used universally so I'll not explain it to avoid confusion).


The Warning Signal is mentioned in passing at the bottom of page 97 of Foster's book but that refers to LMS days.  It appears from a very quick study of Foster that the LNWR did not use warning subsidiary signals although I would be rather surprised of the company did not use the Warning Arrangement in block signalling (probably calling it  Station and Junction Blocked').  The reason for that is no doubt very simple because 'the warning' could be given to a Driver by means of either a handsignal from the Signalman or, when a train was standing at it, by the lowering of the section signal.  Thus in many respects except where it was used frequently it would no doubt be considered a waste of money to provide a fixed signal to do something which could easily be catered for in another way.


So to sum up for a subsidiary placed below a section signal -

Shunt ahead = only go as far as you need to go to carry out your shunting move.

Calling On = go very carefully, there's already another train in the section (so it would only be used reading into a Permissive Block section).

Warning = the section is clear so you can go but you must regulate the speed of your train to ensure that you can stop at the Home Signal of the next signal box in advance because there is something standing in advance of that signal.


I hope that answers your question

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Many thanks Stationmaster, that clarifies it.
I had read that on page 97, but couldn't get my (clear, non-drinking) head round it.
Again many thanks.

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