Steve Fitzgerald wrote:
In message om, Harry
G writes

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.

I'm not aware of any enlargement as you suggest, and I drive through it
most days and suspect I would have noticed by now. Definitely no
platforms down there other than T4, T123 and a set of steps at each
evacuation shaft.

Assuming that this site:
http://underground-history.co.uk/heathrow.php
is correct, it looks like there is an enlargement of the running
tunnel, but as you say no platform. I knew I remembered reading
something about this before (not going loopy!), having seen the site
from trains previously.

There are two emergency evacuation shafts - Sanctuary Road and Wessex
Road (I gather they are both Airside). They are both located between T4
and PiccEx Junction on the loop. Where they go though, we never find
out! I was told during training that they lead to a locked, alarmed
door. Any attempt to leave by this route will find you in the company
of a large number of armed police in very short order.

I'm guessing that the Wessex Road shaft in question comes up somewhere
to the eastern edge of the current T5 construction site boundary, but
as you say it will be somewhere in the middle of the runways and the
taxiways, so remote from any public access. Don't suppose you could be
persuaded to try and find out, given the alarm mentioned and the
consequences ;-)