In message , Roland Perry
writes

It's been said in here several times that T123 is the timetabling
point. However when I was on one such train recently I noticed that the
displays inside the carriage changed to "Cockfosters" at T4.

Depends on where the driver bothers to reset them.

Hatton Cross Heathrow 123 via T4 is 'Heathrow Loop'. The rest is
east/west bound.
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