"Clive D. W. Feather" wrote

The New Lines to Watford Junction, and the Bushey Triangle (to give access
to Croxley depot) were electrified in 1917, basically as soon as the
Bakerloo Line was ready to use it.

The Croxley Green branch was electrified in 1922 and the Rickmansworth
branch in 1927. This is just as likely to be post-war austerity and
dealing with Grouping as any other reason.

The Met Watford branch was opened in 1925, and electrified from the start,
though for the first few months half the service was provided by steam GC
trains from Marylebone. There was also, until 1934, a shuttle service
between Rickmansworth and Watford. When the Croxley link is opened is there
a case for an Amersham to Watford Junction service, as well as Met trains
from Aldgate/Baker Street?

Peter