A stock on the Heathrow branch

The eastbound local line from Northfields to South Ealing is also LU's test track. The train would most likely have been sleeping over at Ealing Common depot, and would have been testing something like brakes or motors or whatever needs testing on the move....

That's a new one on me! My immediate reaction was that sub-surface stock was banned from west of Central because of the mercury tubes but of course they were not there at the time the District still ran to the West!

The test track is actually between South Ealing and Acton Town (E/B local).