RDNA wrote:The problem is that his week I was handed an extract from a book with an illustration of a junction and the locking tappets.....
This states that the points behind the signal are also locked.
What you refer to is 'each way' locking of trailing points in rear of a Stop Signal. The idea being that after a stop signal is cleared for a train to go forward the signalman should not be able to change the position of points behind the train, as this could possibly allow a second train to approach the stop signal before it had been replaced to danger.
So on your diagram -
3 reverse should lock 6,9 and 10 'either way'
14 reverse should lock 7 and 10 'either way'
15 reverse should lock 11 'either way'
Exemptions were sometimes allowed where there was an exceptionally long distance between the points in rear and stop signal ahead and having to wait for a train to pass the signal before, for example putting a crossover normal, would cause delay to parallel moves.
Full sequential locking would of course mitigate against the perceived danger but it was by no means universal, or provided at all by some companies.
Hope this helps and does not complicate your project too much!
DB
To
possibly supplement DB's excellent explanation Brightspark, I believe, from reading a rather long time ago, some of the Institution of Railway Signal Engineers' famous old 'green books', that this form of locking
(not specific to only the LSWR, by the way) was,
I think, referred to somewhere within them under a heading of "Holding The Route", for the reasons outlined by DB.
Plus, I also considered that this locking had an additional potential benefit when there may not have been a second train to be signalled over those points in their opposite position to follow the first, of preventing,
or at least reducing, the risk of the signalman accidentally prematurely moving the points to their opposite position before the ('first') train passed clear of them.
AFAIK, this 'each way' locking became a widely applied principle, though possibly not universally, until means, other than the signalman's eyes, were developed and installed
[typically track circuit(s) ] to positively detect when movements passed clear of such points.
Doubtless though, there were boxes where this was done but the time, trouble and expense of altering the mechanical locking to remove the 'each way'-ing was not seen as justifying being made a pressing requirement for carrying out immediately, and perhaps some such locations were in fact never done.
Ironically that page has been modified since your comment was posted and one sketch now replaced by an actual photo!
