The latest information from the BBC* is that the engineers' train broked
down near Archway while travelling south at 0525, so it looks as if this
breakdown may have been the original cause of the engineering works
overrun. The report says that an out-of-service Northern Line train was
coupled to it and was towing it northbound when the coupling failed.
The engineers' train then ran down the hill southbound.

"LU staff took the decision to divert passenger trains to the City
branch while directing the engineering train to the cleared Charing
Cross branch. The engineering train came to a halt at Warren Street in
central London at 0657 BST because the station is at the bottom of an
incline."

The RMT are claiming that a collision with a passenger train was only
narrowly avoided, but since they are reported as saying that this was at
Archway, it seems unlikely.

I assume that the tripcock arms would have been raised to enable it to
be towed northbound on the southbound track, and hence weren't available
to stop the train when it broke free. The RAIB report will make
interesting reading!

* http://www.bbc.co.uk/news/uk-england-10964766

--
Richard J.
(to email me, swap 'uk' and 'yon' in address)