Sorry for picking this up so long after you posted it, but something was playing on my mind here and I finally figured what it was: As far as I'm aware Preston already has 5 through platforms that are broadly dedicated to through trains on the WCML: Platforms 3, 4, 5, 6 and 7. Off the top of my head, these platforms are used by about 4tph in each direction (2 * London-Scotland, 1 * Manchester-Scotland, 1 * Manchester-Barrow/Windermere) plus a few occasional extras like Liverpool-Scotland, plus freight. That works out at about 2 trains per hour calling per platform. It seems odd if that leaves no capacity to fit an extra 2tph in somewhere, since numerous platforms up and down the country accommodate far more trains than that.