In message . com,
TheOneKEA writes

The plan is to turn all trains on the last day the loop will be
available to us. Anything left over, I'm told arrangements will be
made for a Picc driver (Acton, I gather so no unusual moves for me
:-() to do a special stock move Northfields - Hammersmith - Mansion
House (not sure why they need to go this far?) - Reverse to High
Street Ken, reverse back to Hammersmith to use that triangle. He
will have a District 'conductor' with him who knows that route.

Why go all the way to Mansion House?

I don't know, that's why I asked the question.

Clive Feather states that lineside shielding for 1973TS is only
available on the District between Ealing Common and Gloucester Road.
Unless shielding has been extended to Mansion House, the signalling is
going to go out to lunch if a 73TS is turned during traffic hours.
Why not go via the Cromwell Triangle instead? Or even Lillie Bridge?

Which was my understanding too, although this is what I've been told
will happen.
--
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