On Fri, 11 May 2007 07:02:55 +0100, Clive D. W. Feather wrote:

* From north of Harrow-on-the-Hill to north of Moor Park - four tracks:
Southbound Slow
Northbound Slow
Southbound Fast (no platform except at Moor Park; used by Chiltern)
Northbound Fast (no platform except at Moor Park; used by Chiltern)

At Moor Park, the slow lines connect to both Watford and Rickmansworth,
but the fast lines only to Rickmansworth.

Unless I've misunderstood something, according to a recent post
) and thread, there is a connection
from the fast lines to Watford, and it's currently being moved further
south.