probert1 Wrote:Hi
Could someone please give me a clue as to how I could signal the layout across the single line section over the viaduct. If I was to have a protecting signal at 1300m, prior to the DOWN VALLEY/UP VALLEY junction a train could potentially be standing partially over the viaduct, is this allowed?
Alternatively could the signal protecting this junction be moved back to the DOWN VALLY BRANCH prior to the single line section.
Apologise in advance if I'm asking a noddy question but in my defence I am a novice!
Thanks - Paul
1. As so often is said- you must signal to your railway's practices; without knowing which environment you are familiar with it is not possible to answer clearly.
2. Signalling rules are not like the 10 Commandments in the form of "Thou shalt not....." given from on high. You need to think of them more as "accumulated wisdom" of best practice / avoidance of things which have proved unsatisfactory in the past - that is why accident history (not just of the spectacular crashes) is important.
3. Where "rules" exist you need to understand the WHY the rule exists as much as WHAT the rule is. This is because in the real world you need to bend / break individual items to get the best overall solution. All engineering is compromise between various factors; without understanding you can't hope to get the balance right. According to the precise circumstances the actual "answer" may be different because of slightly diffferent weightings between the factors. Not just in engineering- look at the different classes of different food supermarkets that exist and the various "ranges" each offer and the reported change in fortunes of some of the players in recent weeks as the economic outlook has changed!
4. Any signalling the layout decision needs to come to a balance between "safety" , "operability", "economy". Safety comes first but only to a limited extent; the ultimately safe train is one that is stationary on a track reserved for itself. However that is not useful; unless a railway provides an effective means of transport, sufficiently fast, sufficiently reliable and at an affordable price then people won't use it and travel by other, probably more unsafe, means of transport.
5. Now to the specific (though recognise that the above WASN'T off topic but "setting the scene" as the way in which you asked suggested a lack of comprehension at a higher level). Think of the risk of train stopping on viaduct; is it viaduct collapse or being long term weakened from long term support of the extra weight? is it train being blown off in a gale? is it the fact that a passenger may think that they have stopped at the station and then attempts to get off onto what they think is the platform but turns out to be the viaduct parapet wall? In most cases I think that the last is the most relevant; or rather would be in the dark and where the passenger can open the door (i.e. not power operated, not centrally door locked)- in other words it is probably a risk that existed in the UK when many trains stopped at dimly lit stations in rural areas and there was no door control- far less of an issue now since 2000. Heritage lines (and just a few specials run on NR) do feature such operation but rarely run in the dark and there are certain other mitigating factors; some actually deliberately stop evening dining trains for a long time at such locations so that the clients can have a good view when eating at their tables, but they are scarcely likely to abandon the train in haste during their meal!
However do not forget the poor maintainer who needs to access. If equipment (not just signal but associated track equipment) is on viaduct (where can location case be put?) then how do they get to work on it safely? Almost certainly needs to stop traffic to be able to do so- lose capacity unless long gaps in timetable. Do all maintenance at night?- inefficient and other safety risks. What when need to access immediately because of a fault arising? None are insuperable but all are disadvantages [think of the Underground and how they address these issues]. If placing equipment in such undesirable locations, at least let the examiner know that you have considered and have mitigated as much as you feel circumstances allow.
6. So IF you really needed to, then it would be OK to place a signal at 1300. However it is really stupid place given that you are stopping a train on the botttleneck section- the one short bit of single line on what is otherwise a double track route. What is the advantage of stopping it there?- better to hold prior to the viaduct on the double line portion. ALSO READ THE NOTES- you are supposed to be providng "standage for freight between station C and the commencement of the single line"- this is actually a very tight fit and there has been correspondence on this subject on the Forum. Basically I'd put Down signals as close to the switch diamonds as I could and similarly Up signals at 1400 just clear of the convergence and work out what standage could be provided. Declare this maximum, note that it is non compliant to client's aspiation and state they have choice between accepting it or persuading P'Way to move the single lead closer to the viaduct.
If you did want (i.e. think that the traffic description suggests that it is needed) a signal to allow a second train to be brought up close behind the one(s) stopped on the branch awaiting admission to the station, I'd put its overlap to be at the IRJs positioned at 1400 and the signal an overlap length (hence nominally 180m) from there and thus just onto the viaduct (bad for maintenance but not very bad) provided that minimum braking were respected, otherwise to the minimum braking distance or specify special aspect level controls as a closing up signal.
I hope this illustrates the thought process that you should be going through- signallling the layout is not just a matter of "painting by numbers" and plonking a few signals down on the plan to some simple "black and white" rules that are set in stone!
PJW