You would be perfectly right to do so; indeed it is the traditional thing to do, much of Network Rail's existing signalling is like that and the examiners would probably expect just that as a solution (which I have made clear in the Module 2 IRSE Study Pack).
The reason why I showed as I did is that I was following NR's most recent standard; I am afraid that like many of the newer ideas I cannot provide a convincing rationale; perhaps I am getting "long in the tooth" myself. To be charitable,
1. there is less "additional cost" when the interlocking is computer based rather than requiring additional relay hardware to implement (and indeed on systems such as Ebilock / SIMIS-W each point end is actually a separate entity within the geographical interlocking anyway, however the signal application engineer numbers their Signalling Plan)
2. there is possibly a potential benefit in failure mode situations.
Personally however, if I dictated policy then I would not have changed; I believe that there is still "cost" and I think that almost all the advantages can be achieved by retaining numbering as one three ended set PROVIDED the single lead (722) had a separate detection input to the interlocking than the switch diamond ends.
I believe that NR often sets policy on not much more than a whim and does not properly assess all the options- it correctly identifies that there are problems with some existing 3-ended installations and that they would be overcome by setting a maximum of two ends per point number, yet fails to realise that there is a "middle way". Read "Modern Railways" magazine re "boiling frogs" and the constant ratcheting up of industry costs because of the loss of ability to balance various factors to come up with the most appropriate solution relying upon engineering judgement ....better stop before I get too political and upset my current employer!
For the exam then, feel free to number as you describe, but if you do so then do not claim that your layout is adopting "current NR standards". Perhaps "British Mainline practice, circa 2000" is a better thing to quote for this and other issues.
Hope this resolves your query- if not then post further follow up,
PJW
The reason why I showed as I did is that I was following NR's most recent standard; I am afraid that like many of the newer ideas I cannot provide a convincing rationale; perhaps I am getting "long in the tooth" myself. To be charitable,
1. there is less "additional cost" when the interlocking is computer based rather than requiring additional relay hardware to implement (and indeed on systems such as Ebilock / SIMIS-W each point end is actually a separate entity within the geographical interlocking anyway, however the signal application engineer numbers their Signalling Plan)
2. there is possibly a potential benefit in failure mode situations.
Personally however, if I dictated policy then I would not have changed; I believe that there is still "cost" and I think that almost all the advantages can be achieved by retaining numbering as one three ended set PROVIDED the single lead (722) had a separate detection input to the interlocking than the switch diamond ends.
I believe that NR often sets policy on not much more than a whim and does not properly assess all the options- it correctly identifies that there are problems with some existing 3-ended installations and that they would be overcome by setting a maximum of two ends per point number, yet fails to realise that there is a "middle way". Read "Modern Railways" magazine re "boiling frogs" and the constant ratcheting up of industry costs because of the loss of ability to balance various factors to come up with the most appropriate solution relying upon engineering judgement ....better stop before I get too political and upset my current employer!
For the exam then, feel free to number as you describe, but if you do so then do not claim that your layout is adopting "current NR standards". Perhaps "British Mainline practice, circa 2000" is a better thing to quote for this and other issues.
Hope this resolves your query- if not then post further follow up,
PJW
MarkN Wrote:Hi,
Please could you explain why you paired the switch diamonds, 723a/b but did not include 722 points in the same point identity. I would have made that set a triple ended set of points with the same identity as they are all required to be normal for parallel moves, and all reverse for moves from 724 approaching 720?
Regards
Mark