Paul Corfield wrote:

The boundary point is known as Mantle's Wood. An odd bit of railway
given it's LU property but never used by LU passenger trains - only
Chiltern.

And last time I went up there the boundary between the two was rather
noticeable, as you had an obvious transition from sectional track (LU)
to continuous welded rail (NR), and from LU high density signalling to
incredibly long signal sections. (If memory serves, there are only three
or four signals between Mantles Wood and Aylesbury).

Cheers,

Barry