In message , at 09:19:15 on Fri, 6 May 2016,
Anna Noyd-Dryver remarked:

What I did notice was almost all the pax got off a HEx train arriving at
T23, so it was almost empty for the last part of the journey to T5. I
wonder if T5 BA pax are more savvy, and more likely to use the Tube when
travelling from central London or the western suburbs? Of course, many may
be British and get to the airport by road.

It could be the time of day, and what flights are leaving (balance
between business and leisure flyers). How many subsequently caught the
train to T4?

If flights are evenly distributed between terminals,

The figures for 2014 are the latest published:

Terminal 1 - 9.8 million passengers on 81,696 flights
Terminal 2 - 6.2 million passengers on 41,481 flights
Terminal 3 - 16.6 million passengers on 82,801 flights
Terminal 4 - 9.2 million passengers on 54,567 flights
Terminal 5 - 31.6 million passengers on 207,859 flights

one would expect 3/4 of passengers to leave the train at T23, surely?

Clearly not. But first we have to look at how busy each terminal is
during the day, and what the proportion of business vs leisure
passengers is during the day.

If one terminal has a lot of laughaul flights to business destinations
in the early morning and shorter hall flights to tourist destinations
later in the morning then the percentage HEx passengers for that
terminal could change.

Similarly, while we know the runways are 98% busy all day long, the
balance of which terminals they are flying from could change during the
day. eg Dubai is one of the most popular destinations, but has just one
flight from Heathrow before 1pm, and five in the afternn, and seven in
the evening - 7:50pm and later. Those latter flights, five are from T3
and two from T5, which will skew passengers towards those from about 5pm
onwards.

It may be the case that these sorts of effects balance themselves out,
but they don't necessarily. Remember also that those particular flights
use big planes - 3 x A380, 3 x 777, 1 x A330.
--
Roland Perry