The Piccadilly Line service was disrupted and was initially operating in two sections:

Wood Green, Arnos Grove Oakwood Cockfosters to Kings Cross

AND Hammersmith to Heathgrow/Rayners Lane

It would appear that signalling cable was damaged at the end of an overnight engineering train possession

Quote:

Trackernet's Replayer function shows that the first trains were running with Heathrow and Cockfosters as the intended destinations right up until they were reversed ay Kings Cross and Hammersmith.

The closure was originally attributed to overrunning engineering works, there were no reported problems Sunday night so it looks as if the damage was done during engineering hours and not detected until after the start of traffic

And:

PB13a, PB13b and PB13c signals were failing this morning due to AR track circuit being down yes there was a cable fault but not sure which actual cable was to be at fault 

They could of ran trains through the area but it would of been a painfully slow process hence the reason to suspend They did try a couple but on average it took 15mins to get through the Hyde Park area

If there's a late surrender, trains will still come out of the yard (sidings) with their correct TDs and will run as far as possible with the expectation of the LSP being lifted. Yesterday (Monday) trains were able to get as far as King's Cross and Hammersmith, the closest points to the buffer sections where trains can be reversed.***

We quite often get late surrenders of protection (LSP) due to overrunning engineering work, but they're lifted before trains start to run so outside of Service Control and TAC no one knows about them.

*** Note that  depending on the engineering problem(s) sidings(s) located at the disused Down Street  station between Green Park and Syde Park corner can be used to marshal a train. The destination will shown on the train describer and public timetables as Hyde Park Corner