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.

The second train was the first to leave Acton Town, and I dashed across
when I heard the doors beeping. To my considerable surprise, we stayed
on the District tracks, running non-stop to Hammersmith. The other train
(ie, the one I'd moved from) overtook us along the way, and was leaving
Hammersmith just as we arrived there at the District platform.

I can't remember any previous occasion when Piccadilly Line trains ran
in parallel on both westbound tracks, with both running non-stop. As far
as I'm aware, the District service was running, and I think we must have
been following a District stopper, which is why the other Picc train
overtook us.

Does this sort of thing happen often, and what could have made it
necessary? Or did someone make a mistake?