YorkshireBear said earlier that NR are making 'passive provision' for the bay's future extension alongside the Down Slow to form a loop line. with a new turnout in parallel with the slow to fast crossover. I expect that means building the new track as close as possible to the Down Slow through the station. There is also an opportunity to add one further crossover at Langley Jn for more flexibility.
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Out of interest, how are you making these diagrams?