(22-09-2010, 05:36 PM)alexgoei Wrote: Hello PJW,
I have taken note of your comments and have incorporated them into my 2007 attempt which I just posted this evening.
Have just printed this off after a long day at work that became necessary at short notice; however I have a day off tomorrow and will look at then as I know exams are now very close!
Quote:The salient points I wish to raise are:
Reversal of the Freight train at Down Slow at Platform C. I note your comment about the freight train being routed into the Down Siding and after loading/unloading to draw forward to the Down Slow at Platform C. I did not think about that as a possibility. However what was obvious was that the Turnback Siding was more than 400 metres long and a Frieght train was routed there to await its path it would be out of the way of the UP or Down Main.
You are right that the freight would fit in the turnback siding and could be routed in there. The thing is however that unless it needs to access the siding, there is otherwise no rationale for it to need to reverse at the station at all.
I really think that the examiners worded the notes badly; I expect that they really anticipated all freight passing through, albeit needing to be held just beyond the station until a suitable time for a path on the double track portion. Where there is a siding with no declared traffic you still need to provide a route in and route out with just a little thought for its use, but generally safe to assume that use is rare so don't spend too long on it as it won't be important.
Quote:The consideration for shunt signal 305 were as follows:
Signal 121 in its position 25 metres from the platform would result in the 400m Freight Train standing over points 206A and possibly 205A. While I did contemplate shifting signal 121 forward to point 208A, I thought that its new position (about 130 metres further away), given the curvature of the track, would render signal 121 not visible for a train stopping at platform C. I did consider putting a banner at the platform but decided against it for reasons I cannot now recall.
I now see what you were thinking- a lot of it is valid and your solution would certainly work athough it isn't particularly desireable I think.
In reality we only have one freight per hour so it should always be possible to clear 121 for the freight to have a running move up to 129 to be held there for a path. We don't really want to waste one of the platforms holding a stationary freight, although I agree that permitting the use of the crossover 205 would be invaluable in those circumstances should that be the case.
So you are certainly not wrong- it is only that my judgement is that we don't need to worry too much about the problem on this layout scenario as I assess the likelihood of holding a freight at 121 to be low. It was however a very valid consideration; if you had drawn a line back from 305 along the track showing the 400m standage, then it would have given the clue to the examiner what you had been thinking.
The disadvantage of moving 121 to 208 would hae been that a freight train held at 129 would almost certainly have tailed back into its overlap and thus have prevented a train coming into the station to terminate / reverse; I suppose that you could have provided a ROL to overcome. Moving 121 would mean that 125 should move as well. I am sure you made the right decison to place where you did.
Just be aware that you do NOT need a signal at the end of the platform to tell the train driver to stop; they know to stop at the station and will do so (well except for the very occasional embarassing mistake) even if the signal is at green. Where the station is on plain line and no opposing moves then there frequently is no signal in the vicinity at all, but if there is a stopping hadway requirement to worry about then it improves headway if there is a platform starter.
In the case of station C there is certainly a lot of sense in being able to see the signal from the platform - it would be particularly essential if 122 had been provided with a PL aspect permitting permissive movements in the opposite direction onto a train that is assumed stationary- certainly don't want that train to re-start once having stopped and without a signal to prevent it then that might happen!
However don't assume that the visibility around that S bend kink in the track is really as bad as it might appear- the plans are not to scale vertically and really all that represents is a more gradual widening of the space between the Up and Down Slow to be say 5m apart - the reverse curves probably no worse than would occur when passing through a ladder crossover. You are probably thinking "Metro" where those curves may be very sharp and the line may be in tunnel that would certainly cut off visibility around the curve whereas in the open it isn't normally a problem (though if there are OHLE stanchions, vertical cutting etc then the same considerations apply. In the exam a sustained curve can be assumed to limit visibility but I don't read this examle as being too much of an issue here.
Quote:Actually both Junction D and Station C are supposed to be MAY-FA but what would be a flashing yellow for one may require a flashing double yellow for another. How do I show this on the layout without giving the impression that I did in this attempt ?
Your layout was fine in the Up direction as there was no signal that showed a flashing yellow for one divergence and a flashing double yellow for another; the junctions were as close as they could be but it was enough separation to be ok.
I was suggesting that MAY-FA for the transition Down Slow to Down Fast could also have been provided.
Quote:The LOS 302 is to stop the engine after crossing points 201 so that it can now return to couple with the rest of the Freight train and propel it into the Turnback Siding.
Whereas in older signalling we might have provided such signals, nowadays we don't. The driver is assumed to know what moves are to be made and where there is any doubt (e.g. not a scheduled regular movement) then the signaller ensures that comes to a clear understanding before commencing the sequence of moves. Provision of the signal isn't actually wrong, but we don't do it and would be regarded as excessive by the examiners unless you wrote a specific justification for its provision.
[quote]
3 letter track circuit identities
PJW