Ian Jelf wrote:

Does anyone know why the Aldwych branch was built in such a way as to be
useless? From recollection the southbound branch tunnel is linked to the
northbound mainline tunnel whilst the other two aren't linked at all,
ruling
out both through services to Aldwych and using the branch for reversing
when
there are problem south of Holborn.

Because what eventually became the Piccadilly Railway was actually
conceived as two separate lines which joined together at Holborn. The
Northern of these two had been planned to continue south to "Strand" (as
Aldwych was originally called) and was built anyway, becoming one of those
odd relics of the Tube in the process.

That explains why it was built but not why the connection at Holborn is so
useless. Had it been built as a more conventional branch then it could have
been useful for diverting trains, relieving pressure on the system and as a
glorified reversing bay that the line sometimes needs. But instead any
regular through service (the theatre specials were a late night northbound
service) was scuppered from the point of construction.

In the very earliest days of the line there were through services from
"Strand" (not sure if it had been renamed at that point) to the then
Northern terminus of the Piccadilly to cater for late evening theatre
traffic. These ceased very early on and information about them is
patchy.

It was still Strand - the big round of renamings was during the First World
War. As I understand it the theatre through service was only one a night.

I can just about understand why the station was built in the first place,
but why wasn't any attempt made to make the branch in anyway useful? Not
that a through service would have been handy in the long run, but it could
have been more viable for extensions/creating a street interchange with
Temple and so forth.
Well, there was on at least one occasion and possibly more talk of
extending the branch under the Thames to Waterloo, which would have been a
big help but I'm not sure how serious these plans were.

How far along the Strand did the original Jubilee Line tunnels actually
reach? Had they made it to Aldwych the station would probably be working and
vibrant today.