"Huge" wrote

why was the joint line closed? would there of been a case to keep it
open?
or what about north of aylesbury why was this closed? it would of been a
decent route to keep as a fast line to london as it was mostly straight.
also a hell of a lot of housing has gone up in the area so it may of been
stupid to close it.

The Joint Line (Northolt Junction to Ashendon Junction) is still open as
part of Chiltern's Birmingham route. It is interesting that Chiltern seem to
have taken to referring to their two routes as the Met line (via Amersham)
and the Joint line (via High Wycombe).

IIRC the Ashendon Junction to Grendon Underwood link did outlast the closure
of the Great Central north of Calvert, though was closed between Ashendon
and Akeman Street in the late 1960s after a derailment damaged track at
Ashendon Junction. Akeman Street to Grendon Underwood lasted much longer, to
serve a fertiliser depot.

The closure of the Great Central north of Aylesbury has been discussed at
length, here and elsewhere, but when it closed the remaining stations
between Aylesbury and Rugby had very little traffic, while for destinations
Rugby northwards other routes (WCML and MML) offered much faster journeys.

Housing development which has taken place in recent years north of Princes
Risborough and Aylesbury was not envisaged in the 1960s. The railway has
responded by reopening Haddenham station, increasing service on the Chiltern
Line (north of Princes Risborough there are now 3 trains an hour, while in
the late 1960s it was about 7 trains a day). It looks very likely that a
passenger service will run to a new station a couple of miles north of
Aylesbury to serve a new housing development, and it is not unlikely that
Aylesbury - Calvert - Milton Keynes will be reopened, serving proposed new
housing at Quainton and Winslow, as well as at Aylesbury and Milton Keynes.

Peter