On Feb 24, 3:52 pm, Paul Terry wrote:
In message id, Dr J R
Stockton writes

I've misplaced a better answer; but the unused hole is a bit over one
train-width to the north, and a few yards to the east, of the working
one. The ends are easily seen.

Ah, got it!

When built, both tracks of the Kingston loop buried

burrowed?

under the main line
at Elm Road, and then ran in parallel with (but independently of) the
main line (i.e. on what are the southern pair of the four tracks). So
the mysterious hole was for the Kingston Loop up-line

The up line presumably being Kingston to Wimbledon (worth confirming,
as of course both routes out of Kingston go to London).
Did the rearrangement happen when the Wimbledon flyover was built (or
v.v.) ?