asdf wrote:

ISTM that it would be better to reverse the direction that trains go
around the loop (and still lay over at T4). This would avoid the above
problem, and effectively increase the service frequency to T123 by 50%
(at the expense of effectively reducing it *from* T123, but in that
direction people aren't under time pressure to catch flights).

Slightly more radically, they could even close part of the loop,
running trains either CL-HX-T123-T4-T123-HX-CL, or CL-HX-T4-HX-CL
(where CL = Central London and HX = Hatton Cross). This would simplify
the line map and service pattern.

They should shut Terminal 4 (station and terminal) until T1, T2, T3 and T5
are all operating to capacity, which will be years away.