In message . li, at
20:20:22 on Fri, 20 Aug 2010, Tom Anderson
remarked:
http://www.rtmjobs.com/rail-news/art...-rail-mounted-
milling-machine-trials-on-uk-railway/

Running rails on two stretches of the Northern line – above
ground between Colindale and Golders Green and in the tunnel
from Mornington Crescent to Warren Street – will be milled at
night using the specially developed machine.

Yes, that sounds a lot like what may have been going on.
--
Roland Perry