Being involved with the Station Code Game for a number of years, I am surprised that no-one has mentioned the three NR station codes that begin with "Z"
Nice and simple explanation for those, namely they're LU stations. Though there are, of course, exceptions to that particular rule in the form of the Great Central and Metropolitan Joint Railway stations between Harrow-on-the-Hill and Amersham (inclusive), and the "New Line" stations between Queen's Park and Harrow & Wealdstone (inclusive).
