*The only explanation that makes sense to me is that the
plan was to build some bunkers, in locations that were accessible
(because of the existing stations), with no particular plan beyond
that.

Robin

I think the most plausible answer is that while they were first and
foremost shelters, LT took advantage of their construction as a way to
make a start on the express lines - they were built as running tunnels
on a suitable alignment under stations they didnt expect the express
line to serve. There may be one or two exceptions, but i think its
pretty clear they werent built without an eye to a future use.

Chris