(30-01-2011, 06:51 PM)interesting_signal Wrote: A question about the 1999 aspect sequence in the Up direction.
The model answer shows 116 being approach released. I don't quite understand why. I would have expected 118 to show a green when 116 is at yellow. There are no distances given so hard to make assumptions. There are no notes on the layout, so I don't how you would determine that AJ needs to be occupied before 116 displays a yellow. Anyone know why this is the case?
Neither do I.
I agree, just can't see why 118 wouldn't show G up to 116 at Y to 114 at R.
As you say would only need to do this if 116 were underbraked to 114, but can't see why it would be placed in such a position; it isn't good signalling and modified 3-aspect sequence only done in extremis and there just is not justification for it here. Nothing on the diagram to give any hint of required braking or distances, so of course you'd assume that there was braking- why wouldn't there be?.
Hence I think you are dead right and I regret you have been led up the garden path by the "model answer" (which I must admit I hadn't looked at before). However full marks for recognising!
What is depicted appears to be the sort of practise more akin to mechanical signalling (118 as the signalbox's distant) that just sometimes remains implemented when semaphores are replaced by colour lights but still operated off the same lever frame.
This contrast with true MAS which the diagram certainly implies; however if you look very carefully you'll note a lack of train detection beyond 123, but there is no associated note that that signal is released by Line Clear, no note that the line beyond is operated under Absolute Bloc. On this assumption that the colour lights we see depicted are in fact replacementts for semaphores, then there is unlikely to be braking between the Outer Home and the Inner Home, so the Distant would continue to apply to both and thus could only show Green when all the associated stop signals in the signalbox area are cleared. Quite how a student is meant to tumble to the significance of this (and indeed make some pretty unwarranted assumptions on the basis of the flimsiest of evidence) when attempting to do the opposite direction aspect sequence is completely beyond me; I only thought along these lines when trying to rationalise why the model answer is as it is- and not under exam time pressure).
My theory is that perhaps the sequence reflects the situation which atually existed at Grosmont which I am pretty sure was the inspiration for this layout.
The Railtrack line to Whitby is the single track in the direction of BB.
The NYMR from Pickering terminates in the station (in actuality a single track apart from the parallel line through the tunnel to the engine shed). In those days there was no through passenger train running- this layout seems to have been ahead of it time in that respect. Of course there are no colour lights signals really there, but a fine selection of historic semaphores.
Whereas the above may explain why the aspect chart was drawn as it was, it really is inappropriate and far from helpful to the student as a model answer.
Anyone else feel they can come to the defence of IRSE and prove us both wrong?
