Tim Roll-Pickering:
Does anyone know why the Aldwych branch was built in such a way
as to be useless?

I've never seen an explanation of that. Clive Feather has never seen
an explanation of that.

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 ...

Right. In addition, there was no crossover north of Holborn on the
main line that would have allowed the branch junction to be worked
as a "single-lead junction" even if they'd wanted to; they put one
at Covent Garden instead. Nor was there a crossover at the south
end of the branch, only near Holborn on the branch. Here's an ASCII
version of the diagram in Rails Through the Clay 2nd edition (RTTC2):

to Finsbury Park
| |
| |
| |#
* |#
/| |#
/#| |#
HOLBORN /##| |#
/# #| /
/# #| (
/## #|
_/ # # = #|
__/ # | |
__/ _ ) # | *
__/ __/ | /|
_* __/ |/ |
/ |.__/ * |
/# * | |
/# #/ | |
COVENT GARDEN | |
and on to Hammersmith | |
| |
| |
| |
| |
STRAND # | | #
(- Aldwych) # | | #
# | | #
= =


Ian Jelf:
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...

Tim Roll-Pickering:
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...

Exactly.

RTTC2 says the branch was originally worked using only the east track
off-peak and with a train on each track working independently at peaks.
By 1912 it was down to a single shuttle at all times, using the branch
crossover, and in due course the other tracks were lifted and this
became the only possible route.

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...

Yes, it changed in 1915.

As I understand it the theatre through service was only one a night.

Before the branch opened, this train started from Holborn and ran express
to Finsbury Park, calling only at King's Cross and Holloway Road. It was
then altered to start at Strand (Aldwych), at 11:13 pm (later 11:28).
From 1908 it called at all stations. [RTTC2] I don't see anything to
say when it stopped running.
--
Mark Brader "Remember, this is Mark we're dealing with.
Toronto Rationality and fact won't work very well."
-- Jeff Scott Franzman

My text in this article is in the public domain.