From the Wikipedia page on the Victoria Line:
Northumberland Park Depot is the service and storage area for trains on the Victoria Line of the London Underground, the only part of the line above ground.
Trains access the depot by a tunneled branch line to the north of Seven Sisters Station. Opened with the first stage of the line in 1968, the depot is next to Northumberland Park Station, on Tottenham Marshes, Tottenham in the London Borough of Haringey. As part of Transport for London]]'s tube upgrade scheme, the depot has been expanded and upgraded to accommodate the new fleet of 2009 Tube Stock trains.