If you look on Google Earth you can see the crossover 'south' (more
like east!) of Harrow on the Hill. If you really zoom in you can just
about see that on the London side of it, the Chiltern tracks don't
have the 3rd & 4th rails.

I grew up in Chorleywood and can just about remember the Master Cutler
thundering through (it scared me), and having to wait at Rickmansworth
for an engine change to go to London. At that time there were coal
yards and I think a loop at Chorleywood but those got turned into a
car park, and the overbridge was eventually removed. However the line
was then electrified out to Amersham, the engine changes were no more,
the sidings were built at Rickmansworth (plus I think the bit of
bypass north of them), and I do remember having to be careful which
train I got on going to school as there was one in the morning that
was non-stop from Chorleywood to Marylebone.

When getting a train to London at the weekend I liked to get the
Marylebone service and get a seat right at the front, as usually the
driver didn't have the blinds down so you could see the track ahead.

I also remember when everything was frozen one winter (very late 60s
or early 70s I think) a pannier tank turning up from Neasden on points
de-icing duties. Some time later a diesel turned up and I stood in
the guard's compartment (as did lots of other people) for a very slow
journey to Chorleywood. I don't know how long it took as it was dark,
but it was certainly not the usual 4 minutes.