On Apr 2, 9:08 am, Boltar wrote:
How far could District line trains head down the heathrow branch of
the piccadilly line before they're out of gauge? Do they ever go a
short way down any more to use the test track for example?

B2003

The last place they could reverse would be Hounslow Central.

There is a "Surface Stock Detector" after that would maintain the WB
Advance Starter at Danger, should the train wish to be de-roofed!

In reality, if a D Stock did accept a wrong signal at Acton Town, it
would be reversed at Northfields Depot, though if the depot can't be
used a trip to Hounslow would be needed!