"Dave Arquati" wrote

Why was it that Amersham ended up as the terminus in the first place? I
know it's to do with the electrification scheme but I can't remember why
it didn't just run to Watford, leaving Rickmansworth northwards as
BR/Chiltern.

The Met was electrified to Rickmansworth in 1925, the year the Watford
branch was opened. In view of the 2-track bottleneck between Harrow and
Watford South Junction, Rickmansworth was the first convenient place for
traction changes on Aylesbury trains to take place.

Quadrupling between Harrow and Watford South Junction was proposed in the
1935 London Transport New Works Plan, but this work, and electrification to
Chesham and Amersham did not begin until 1959.

So why was Amersham chosen as the limit of electrification in 1959? Good
cases could presumably have been made for electrifying through to Aylesbury,
with all remaining Great Central services being diverted via the Joint Line,
Ashendon and Grendon Underwood Junctions, or for handing the new fast lines
from Harrow South Junction to Watford South Junction, and on to Amersham
over to BR, not electrifying them, and serving all stations Rickmansworth to
Chesham and Aylesbury with dmus to Marylebone.

Peter