[Assuming you mean "eastbound"...]

"Recliner" wrote in
:

I was on a westbound Piccadilly Line train from the Rayner's lane
branch at about 6:45pm on Friday that stopped for a while at Acton
Town, waiting for a relief driver. So far, nothing unusual, but then
another westbound Piccadilly train pulled into platform 4 (the
westbound District platform). Normally only trains from Ealing Common
do this; I don't know which branch the second train had come from. It
would be surprising if another Rayner's Lane train had been following
our train so closely, but very rare for a train from the Heathrow
direction to be using that section of (test) track.

I often travel east from Northfields at that time of day (well, any time
between 5:30 and 9:30), and it's not at all unusual to end up on the
outside (District) platform). I would estimate it happens to me more
often than once a fortnight.

It *is* unusual to then run down the slow line to Hammersmith - I can't
remember ever being on a train that did that. Are there points between
Hammersmith and Barons Court that put the train back where it should
have been?