I make brief mention here of the Test Track that's located between South Ealing and Acton Town. It actually uses the east bound 'local' road which, although located next to the Piccadilly Line 'fast' road between these two points, is not routinely used by the 'Picc'.
When in use for testing purposes, the Test Crews take a 'Possession' of the area, which allows them to run the train being tested bidirectionally. The trains themselves are operated by Test Train crews who are qualified on all London Underground stocks, and across the whole of the network too, which enables them to move trains to and from the area, wherever they may normally reside.
From time to time most London Underground stocks appear in Ealing Common Depot, and they locate themselves there for the purpose of using the Test Track - most frequently for the purpose of brake testing but, as with the refurbished D Stock as previously illustrated, it's also used to carry out recording and measurement for a wide of variety of reasons and to assess performance after significant modifications have been made to a type.
About the only type which only appears rarely is the 1973 Tube Stock, as these are kept in Northfields depot anyway which obviously has easy access to the area
Chris Cobley's been out again! This time he hunted down an A60 that was at Ealing Common for the purpose of using the test track and has sent the following collection. As you will see they are in some interesting places, other than just on the Test Track itself! I would emphasise here that Chris is a member of London Underground staff, hence some of these (and indeed others of his photos) are taken from places not accessible to the public.