J Lynch wrote:
Times may well have changed and this may not be an issue now, but one other
potential problem is that this also severs the Northern Line from the rest
of the underground network? The only connection between the Northern and the
rest of the underground is provided by the Kings Cross loop which is
(presumably) out of use for the time that the Piccadilly is split in two
like this? This was probably more important when Acton was the Central Works
but it also prevents engineers trains etc from reaching the Northern, which
could make track maintenance etc more difficult.

Chances are that if they really wanted to run engineers' trains, they
could -- the Kings Cross Loop from the Northern Line connects to the
eastbound (northbound) track on the Picc, which so far as I understand
is undamaged. Hence, an engineers' train from Lillie Bridge to the
Northern Line could run as normal to HPC, then let the batteries take
over to get it to King's Cross. Assuming somebody can operate the
points, it can happily then reverse to Euston.

On the way back, things will be dicier since it'll have to run
wrong-line from KX to HPC (usually it would use the KX crossover, but
that leads directly into the crash site) but an engineers' train can
surely run in a full possession, and then you can do pretty much
anything you like.

So, short answer: the Northern isn't cut off. Nonetheless, I wouldn't
expect much to happen for a little while. They'll use that method if
they absolutely need to get to the Northern, but I don't see it
happening any time soon.