Robin9 wrote:

I'm no authority on London Underground's work systems
but I was always under the impression the Drayton Park/
Moorgate service was operated as a self contained entity.

Obviously servicing and maintenance had to be done at some
other location so empty stock movements were bound to occur
and have no great significance.

As a minor line, it probably came under the management of another line,
presumably the Northern, just as the Drain does today. But, as you say,
that wouldn't necessarily affect stock maintenance and transfers.