axle counters CT Reset / Restore

[PJW]

Peter

You are correct in saying yes I need it for my work. I am talking about EPR/STR and aspect restriction. I have looked at Newport and Edinburgh Waverley as an example but I am aware that the principles used are slightly out of date. I have also read 30080/30081/30082. These just do not go into the level of details I am looking for they are all very open statements which can be interpreted many ways.

Eg what happens with distant auto sections etc?

Jerry

I have an understanding of what EPR STR. conditional and unconditional reset are but only have a minimal knowledge when it comes to the principles behind them.

Any help or anyone who can point me in the right direction would be much appreciated

Oh the joys of non-prescriptive standards!
Another thing about railway standards nowadays is that there are so many different ones to choose from!

If you haven't already discovered them, suggest you look at the attachments in this module 5 post ; perhaps not really what you are after back some useful background.

I also attach a summary of reset / restore produced for a YM exam event a couple of years ago that possibly only covers what you already know, but may be useful to you or others.

I think it is fair to say that we have yet to achieve stability on a satisfactory methodology.  It was the West Coast Route Modernisation that really set the standards of which you are aware, but yes there have been changes since in the light of experience / different projects and contractors getting involved. There is a tension between trying to make sure that
a) the maximum protection is given against inappropriate reset and
b) being able to restore the railway to normal working quickly and easily following any disturbance.

Not only that but there are definite complexity costs in implementation- when this functionality is actually making serious inroads into the capacity of an electronic interlocking then one must wonder if the tail is wagging the dog!

Newport is I think one of the most recent projects and therefore I'd certainly expect it to be different from Edinburgh for this reason, as well as the fact that there is a definite difference in the environment between long sections within plain line railway and short sections in complex areas and slow speed running.  In addition Newport attempted to use a WESTLOCK to implement the size interlocking that would have taken 4 SSI; whereas that was a reasonable thing to do on the basis of a simplistic assessment, there are actually a host of different potential constraints.  An SSI has the capability I think of 1024 latches (including the specific ones used for subroute and suboverlaps) but when WESTLOCK was being designed it was before the days of axle counter complicated reset-restore so consideration of the historic usage of various elements over many SSI schems suggested that this was never the constraint and hence the optimum tradeoff was a reduced provision of latches to increase other elements that were often at a premium historically.  This meant that the designers then found that they had put too large an area into the one interlocking and had run out of latch capacity; I don't know the details but I wonder if this caused some re-evaluation of what was really essential .........

Therefore I certainly wouldn't like to give specific advice re completing Control Tables for them; I'd expect the project to have defined some standards dictating how they are to be completed.

In essence the idea nowadays is generally to allow the signaller a) to reset the axle counter on their own authority, not involving a technician (as per cooperative reset),
b) for this reset to take immediate effect (rather than being a preparatory reset that only restored the section after it had been traversed by a train).
The reason for this is that immediate clearance of the track section permits points to be moved that would otherwise be locked. However we are not comfortable to permit a train then to traverse the section at high speed; there is a small but finite risk that the section was not actually clear and the reset was mistaken- the likelihood may be small but the consequences could be very high and thus the risk assessed to be unacceptable.  Therefore the mitigation of not allowing the signal to clear to a yellow or better; given the fact that there are no POSAs provided in the vast majority of situations then the signal has to be held at red and the driver verbally authorised to pass it.

Because the track section is itself showing clear, then in order to prevent the signal from clearing an additional type of locking has to be imposed.  Basically each path over each train detection section is given a latch memory to register that the section had been reset and there has yet to be a train passing over it and therefore can't really be 100% certain that it really is clear.  Once a train has passed then this element of Aspect Restriction is removed, but for a section with points there is still a possibility that the portion on the other lie of those points may actually still be harbouring an invisible obstruction so there has to be separate memory for that portion which won't be removed until there has been a train over that portion as well subsequently.  

There are various rules that say if the spur section is short enough / visible enough then it can be deemed to have been swept by a train taking the other lie of points, but in general separate trains will be needed.  Then there is the possibility that although a train enters a section that it might not fully traverse its length since there may be a signal mid way at which is could have reversed before getting to the end.  Since Aspect Restriction is normally taken off as the section becomes occupied, then there have to be special controls in such cases that this does not happen, just in case the train doesn