alexgoei Wrote:Perhaps you can show us how you would enter Point 212 on the Points CT.
It would also appear that the point-to-point arrangement appears to be very popular with the examiner and is found in the 1999 and 2000 papers. In fact the 1999 paper on the two points to be completed was a slip with a crossover on one side.
I have made comments on yours to facilitate the clarification process and these are found in the appended file.
Look forward to your comments
See attached which I hope clarifies my explanation which I think confused you as you seem to understand the locking itself.
Re 1999
A double slip is almost always associated with single ends on the adjacent lines. Be clear why we occasionally use them is to save space (length). Assuming need to cross from signal 119 to either of the lines to its left and from 117 to the Branch, this could have been achieved by the simple crossover 304A/B and thereafter another simple crossover 305B/C- however the successive ladder would have reduced the length available for siding 1.
A double slip allows the two crossovers to be overlapped to an extent- the cost is lower speed, more complexity, higher maintenance burden etc. but the gain is the ability "to fit a quart into a pint pot".
For similar reasons note the 301 / 302 arrangement. This has the same functionality as a facing and then a separate trailing crossover; but there would not have been space to fit both of these in within the length available between tunnel and station platforms. Providing a scissors (with its mini diamond crossing at its centre) has many disbenefits but the overriding advantage of getting the required layout flexibility in the confined space.
This is another classic case of point-point- you could never want both points reverse simultaneously.
If wanted to show on CTs then I'd put in Remarks column for 301 called N to R: "Requires 302N" and conversely on 302's CT for call N to R: "Requires 301N".
However I'd advise NOTactually to implement point-point, but JUST USE AS TOOL to help you recognise the point calling by routes that you might otherwise miss.
In this case ensure: ALL ROUTES OVER 301R CALL 301N and vice versa.
If you miss this out then you'd be allowing (unless very cleverly realise the situation and thus add in some clever route locking entries- which is not actually as good when you consider degraded mode operations) both 108A(M) and 107B(M) to be simultaneously set (would not otherwise have incompatible point calling requirements and unlikely to "see" as an opposing route); this would set things up for a collision at the scissors crossover!
So look for the tell-tale signs associated with two point ends within the same track circuit- not 100% definite that there needs to be some FLANK / TRAPPING / PARALLEL MOVES / SCISSORS point calls but probably 9/10 there should be so certainly ought to initiate a warning alarm to focus on the area with such in mind.
PJW