They weren't energised fully to City Thameslink for quite a while afterwards though. IIRC the Network Change Notice to turn both lines on wasn't completed until after they stopped running certain DC units, that caused a gauging issue?
Seems it was all energised in 2012, we had one of those involved discussions in this thread:
http://www.railforums.co.uk/showthread.php?t=59531&page=3
A number of posts came up with all sorts of wibble suggesting northbound changeover in City Thameslink would never happen. However, as I've now been reminded having read the NCN the problem is that a failed train at Farringdon northbound cannot be recovered when the 24 tph timetable is running, because the following train fouls the crossovers between Farringdon and City. That is the fundamental reason behind normal changeover at City T/L.
The Network Rail 'party line' is in this NCN:
http://archive.nr.co.uk/browse docu...005 completion of overhead line extension.pdf