Answers to 2002 Part A Q1 & Q2

[PJW]

Hello Peter,

I have reviewed your comments for the 2002 paper and have some further questions which I have entered the text in red in the appended file.

This is in particular to Comprehensive Approach Locking.

The purpose of 114's approach locking is to continue to reserve the route beyond the signal in crcumstances in which a signaller replaces the aspect which a train driver may already have seen or been given information about at an earlier signal.
118 will change from Green to Yellow if 114 is replaced. If a train is on BR then (note distance signal 122 to 118 is only 340m) the driver would be able to see the aspect; hence tracks BP and BR must be included in 114's CAL if:
a) 118 could itself have ever displayed an aspect other than Red (hence check for it being ARAFOAL), AND
b) the train on BR is actually relevant to signal 118 (hence the condiion on 206R that tells us that any train there must be going towards 116 rather than 118). [of course if the read-through situation is bad then to avoid driver confusion signal 118 should have been held to danger unless BP occ or 206N, but that is a different part of the interlocking story]
Obviously we also need to consider the situation when 118 is ARAFOAL but 116 is not and the situation is nearly the same- however given the nature of 116 and the slow speed over 206R then I'd expect 116 to be MAR on AP occupied (see route box re aspects displayed for 122B) and thus could just test 116 to be ARAFOAL, though since have no route box that positively tells us that then taking the CAL back to AP seems better.