PJW Wrote:For the exam then, feel free to number as you describe, but if you do so then do not claim that your layout is adopting "current NR standards". Perhaps "British Mainline practice, circa 2000" is a better thing to quote for this and other issues.
Three is not outlawed by current NR standard. 11600 part F3.5.1 says (in the bit about devices operated together having the same number) "Such and arrangement should, wherever practicable, be restricted to two point ends in order to simplify testing and corrective maintenance" further saying that you can go to three ends if it is "particularly expedient" and if the infrastructure controller agrees.
I agree with Peter that an arbitrary ceiling of two is a bit low, clearly a quick look at the SMTH permutation table for four, five or six common ends for the detection testing matrix starts to tell you this is a bit OTT.