In message , Brian Watson
writes

As there would be no terminus there would be nowhere for the trains to
make
up any time and this would decrease reliability.

So how does this work now at the Heathrow end of the Picc or, come to that,
on the Circle line?

Heathrow 123 is regarded as the terminus. Trains are booked for about a
7-8 minute stand there.
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)