In message , asdf
writes

If this is the case there could be problems if all passengers in
central London who want to go to T123 & 5 (when it opens) rather than
T4 wait for the appropriate train - the T4 loop trains could end up
rather empty and the others rather full.

I seriously doubt there are enough Heathrow passengers for it to cause
significant problems.

I would concur, although the opportunities to confuse passengers are
endless and should be avoided if at all possible.
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