Clive D. W. Feather wrote:
In article , Peter
Frimberley writes
If I recall correctly, the T4 loop does include a straight section
somewhere near where, at the time of it's construction, they expected
a future T5 to be. However the T5 that's being built is considerably
larger and in a different place than the T5 foreseen back then.

Correct.

AIUI there is a short enlargement (approx 30ft length) of the running
tunnel to station tunnel size on this section, with (possibly) a
platform, but certainly a passageway leading to an access shaft. I'm
not sure if the shaft contains an emergency exit (possible, given the
interstation distance T4-T123), or ventilation equipment, or is just
left over from construction of the loop. This can be seen on the west
side of a train (i.e. left) looking out around halfway between T4 and
T123, and is (or was, pre-closure) quite brightly lit.